# Paralleling Batteries



## dave.verry (Apr 5, 2006)

There is a major disagreement on paralleling batteries on this forum. As an engineer who parallels batteries for a living and designs & builds chargers for them, I’ll give you one guess where I line up.

This conflict has hijacked more then one thread, so probably deserves a posting of its own. Here goes!

I have been considering two states of the battery bank while answering these posts;

1. Batteries on a charger and charging or,
2. Batteries off a charger and discharging into a load. 

It was pointed out that there is a third state where the batteries are off the charger and are not discharging, i.e. no load.

Examining each state with a simplified battery bank of two parrelled batteries, both rated at 100 AH (amp-hours), and one “good” at rated capacity and one “bad” with only 50% capacity or 50 AH. We will have to assume good connections, since everyone does monthly preventive maintenance every month on there battery bank, right? We will also have to assume the “bad’ battery does not contain shorted plates. The loss of capacity will be due to plate corrosion, sulfation and/or electrolyte loss (drying-out).

Under charge, the batteries behave independently, each charging to there maximum capacitance. The “bad” battery will charge faster, given the same energy input, but with two in parallel, the amount of current may be limited by the “good” battery, depending on the charger. Once the float voltage level of the batteries has been reached, both batteries will be at approximately 80% state of charge and the two batteries will continue to charge the remaining 20% independently. Using the 80/20 rule of thumb, 80% of the charge takes 20% of the time, the final 20% taking 80% of the charge cycle time. Because of the limiting effect of the good battery, both batteries will reach full charge state at about the same time. (Assuming full discharge was the initial state of both batteries. You can start without full discharge and the charge time of both batteries may differ.)

At the end of the charge cycle, both batteries will be at full charge, one at 50 AH and one at 100 AH. 

During discharge, the “bad” battery will discharge 1/3 of the current level of the “good” battery. For a 10 amp load, 3.33 amps will be coming from the “bad” battery and 6.66 amps from the good battery. Both batteries will be at the same voltage level and because of this discharge the same percentage of capacity. Both batteries will be at the same state of charge during the discharge.

If the batteries are left off the charger they will drift down to their open circuit voltage after several hours. If the specific gravity of the electrolyte has not undergone change, i.e. the loss of capacity of our bad battery is due to plate corrosion or drying-out, the open cell voltage of both batteries should be the same for given state of charge. (close enough anyway). Since there will not be a potential difference between the batteries there should be minimum to no current flow between batteries. Any difference will be equalized once the batteries stabilize any specific gravity differences. This will be done by discharging the battery with the higher specific gravity and charging the battery with the lower specific gravity. There is no guarantee which battery will be discharge as the one with the lower capacity may have the higher specific gravity electrolyte. In any case the overall system capacity will remain constant. The only loss will be a small resistive loss due to current flow (negligible compared to system capacity).

If the “bad” battery has a low specific gravity electrolyte due to sulfation (this is the only cause I can think of in a battery system) the parrelled batteries will once again try and equalize the open cell voltage and the “good” battery will continue to “charge” the “bad” battery until the specific gravity of the electrolytes are the same. However, any charge taken from the “good” battery will be stored in the “bad” battery minus resistive loss again. Again the battery system capacity should remain constant.

If any work is done by the system, i.e. hydrolysis of water within the electrolyte in to hydrogen and oxygen, that energy may be lost from the system. This should occur, especially in a system off the charger, only when there is physical damage such as shorting of a cell of one of the batteries. In this case the “good” battery will OVERCHARGE the “bad” battery causing electrolysis and gassing and possibly venting. This energy will be lost to the system.

Basically the conservation of energy prevails. Energy from one battery may move to another (current flow between batteries) but the overall system energy will remain the same (discounting the negligible loss from resistive heating). 

Does this mean that you should not replace all of your batteries if you find one bad? Maybe, or maybe not. Only a load test of the batteries can answer that question and that is beyond the means of most sailors. Good practice for reliability is to change all of the batteries at one time. This helps insure that all batteries are in good condition and that you have the battery system capacity you think you do.

Comments please!


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## btrayfors (Aug 25, 2006)

Dave,

Very interesting post. Thanks for making this a separate thread.

I'm a bit dismayed and not a bit disturbed by some of the comments and "advice" I've seen on the subject of paralleling batteries. In particular, to state that this is "never a good idea" is pure poppycock.

For many cruising sailboats, particularly the larger ones, paralleling house batteries is the only practical solution to obtaining the needed AH capacity, where both cost and space considerations prevent the use of large industrial batteries. For many sailors, a large house bank constructed from, e.g., Trojan T-105 golf cart batteries (225AH 6V) in series/parallel is the most cost-effective solution.

Furthermore, the idea of separating your house batteries into, e.g., bank 1 and bank 2 and using them independently is not good practice. Paralleling all your batteries into a single large bank has numerous advantages, including faster charging and longer life. A large capacity bank may be charged more rapidly (more AH replenished per unit of time) than a smaller capacity one. This means shorter run times for your engine or generator, with attendant savings.

The percentage of discharge on a large capacity battery bank per unit of time as a result of the typical house load (lights, instruments, refrigeration, etc.) will be less than the percentage discharge caused by the same load on a smaller capacity battery bank. Typically, this will mean that the house batteries are drawn down less when banks are combined than they are when banks are used separately, resulting in longer life (more discharge/charge cycles) for all the batteries. 

Finally, all house batteries are treated identically in the combined pattern. If they are treated well, their life will be greatly extended and overall costs will be reduced.

Starting batteries, of course, need to be treated separately. I favor an approach wherein a large capacity alternator/smart regulator charges the combined house battery bank, and a small EchoCharger is used to maintain the starting battery which is completely separate. All battery charging devices (shore power, onboard generator, wind powered generator, solar panels, etc.) also charge the combined house battery bank.

Batteries on sailboats don't die. They are murdered by bad installations, improper maintenance, and ignorant practices.

BTW, there's no such thing as a battery with no discharge. They all are self discharging, even with no load attached. Just a matter of rate!

Equally, they are all sulphating. From the moment after manufacture when sulphuric acid is added, they begin sulphating. How far and how fast this chronic process progresses depends on how the battery is treated. Proper 3-stage charging will help minimize sulphation. Failure to fully charge a battery and/or letting it sit without charging for any appreciable time contributes mightily to sulphation.

Just my $.02 

Bill
S/V Born Free


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## dave.verry (Apr 5, 2006)

*Thanks Bill.*

Bill,

I agree with all of your comments. There are a lot of things going on in batteries and battery systems and I was trying to keep it simple to start.

The number one killer of batteries is the lack of proper charging. A lot of posts I have read indicate that the engine is run until the batteries reach float voltage and then turned off. That is only giving the batteries an 80% charge at best. Other posts describe symptoms that indicate bad connections in the system which will also prevent proper charging.

Hopefully others will put in there two cents. Maybe we can start a fight club for electricial engineers.


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## sailandoar (Mar 20, 2006)

*Very good suggestions, thanks!*



> *Paralleling all your batteries into a single large bank has numerous advantages, including ....... *


This point is also made by Nigel Calder:

Boatowner's Mechanical and Electrical Manual








AMAZON: ($39 new // $25 used)
http://www.amazon.com/Boatowners-Me...ef=sr_1_2/104-6103608-1825510?ie=UTF8&s=books


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## hellosailor (Apr 11, 2006)

Well, Dave, I guess I'm the Devil's Advocate here.<G>

GE, in their old industrial battery book, says right out that parallel batteries can work perfectly well. But then goes on to say they will work best when each set of CELLS is paralleled, and then the parallel banks of cells are connected in series. (i.e., three cells in parallel, six banks of them, to make 18 cells combined in the final battery. Not, two six-cell batteries put in parallel.)

I would guess that they suggest this because it "plays the odds" better, averaging the capacity of each group of cells rather than playing one group of six against another, where the variations in SIX cells accumulate in each battery.

But, as you note, there are real life considerations to be taken view of and I suspect it is the real life variations versus the pure ideal where the problems can arise. Or, be eliminated by simply not paralleling.

So, I'd like to suggest some investigation along these lines:

Consider your case of two 100AH batteries running in parallel from one charger, which for systems of that capacity usually means one 60-90Ah automotive alternator with integral automotive regulator. I'm not sure where to begin quantifying anything that specific. We know that the regulator will throttle back the charging amperage when it sees about a 90% charge, and we know that there will only be one charge sense lead, which is reading the average of the two batteries. So we can begin by noting that when the AVERAGE state of the two batteries has reached 90% capacity (80, 90, whatever, that's another example of the differences in specific equipment) capacity, the system will go into trickle charge mode. Which is a problem regardless of battery configuration, but even more so with parallel batteries I'd think one of them will be cooking (boiling out electrolyte and overcharging) while the other is undercharging more than it would alone.

Or don't you think this "charge abuse" will be greater for two batteries in parallel than it would be for separate banks?

Have you actually measured or metered parallel batteries to see the charge states they reach, versus the same batteries each separately charged?

And, given the variation in batteries these days, have you compared mutiple batteries "off the shelf" to see how closely they do or don't compare in voltage and capacity, even when they are twins off the same line? (Assuming twins stay together all the way into the boat?)

"...but with two in parallel, the amount of current may be limited by the "good" battery, depending on the charger." Current limited by the good battery? Not when there's just the one charge sense lead, reading the average of them. Of course, as soon as that average (pulled up by the good battery) reaches a set point, the *voltage* produced by the alternator will be dropped and the current reduced. But AFAIK in the common automotive alternators, they may be limiting voltage or current, there are a number of different schemes and float ranges.

And then, at what point, at what size of battery banks, do we assume boat owners have switched to real marine outboard regulators and real marine high power alternators?? Because at that point, charging can or will change, won't it?

"Because of the limiting effect of the good battery, both batteries will reach full charge state at about the same time. (Assuming full discharge was the initial state of both batteries. ..."

Ouch! Assuming full discharge?! That's outright abuse. Who ever recommends taking batteries below the 50% discharge point? Makers may claim they get the most "lifetime power" out of a battery by cycling it to 40% or 70%, but they all seem to center on 50% and they all say a full discharge can kill a deep cycle battery at least 10x faster than any lesser charging state.
And reaching a state of full charge? Well, again, unless you've been motoring all day, or using a marine regulator, the more common automotive type won't get you above a 90% charge for hours after that point. If you're just running the engine as much as you need for recharging, and shutting it down after that...odds are you'll never see more than a 90+% charge.

"During discharge, ... Both batteries will be at the same state of charge during the discharge." Is that from theory, or observation? At what discharge rate?

And I think it really should be pointed out, the case of cruisers who are regularly cycling their batteries with daily cycling, is going to be very different from the typical weekend sailor--where the batteries may be sitting (one hopes OFF) for five days, then sitting in parallel but feeding only a small load, like the instruments, for six or eight hours during the day.

"If the batteries are left off the charger they will drift down to their open circuit voltage after several hours." That's too generous. I've seen battery makers who suggest that it will take 10-24 hours for the electrolyte to equalize out the charge in the battery, not "several" hours. With an older 12v battery, I've seen that initial charge burn off overnight but then continue to settle, visibly, for longer.

"If ...the open cell voltage of both batteries should be the same for given state of charge. (close enough anyway). Since there will not be a potential difference between the batteries there should be minimum to no current flow between batteries. " But that's a gross assumption. Unless the batteries come from the same maker and the same production batch, the alloys used in them can differ. Different makers and lines intentionally use different alloys making that the greatest problem, but battery making is not watchmaking, mechanical differences in assembly are normal.

"Any difference will be equalized once the batteries stabilize any specific gravity differences. " I'd been told that's not the case, and that is the problem. They WILL equalize, yes. And then as each battery is different, and has a different rate of self-discharge (among other things caused by the physical changes and sulphation happening differently in each battery), as soon as they have equalized they begin to drift apart and the current loop between them starts to flow again. This process does not stop, it continues and it drags them down.

I suppose the simplest way to test this would be to take two "good" batteries, put them on a very smart charger to make sure they are properly charged, and then record their discharge rates over the next month. Then repeat the process, alternating between batteries that were in series and in parallel. Perhaps using two sets to get better data. (What, a month if too long? OK, two weeks? One week? How about run the month, and let's see where the data cross?)

I'd volunteer to do that, but don't have the resources to do it.

"However, any charge taken from the "good" battery will be stored in the "bad" battery minus resistive loss again. Again the battery system capacity should remain constant." So you're familiar with the endless looping...but isn't that the catch? Just resistive loss? Can we put a number, a percent, on that? Again, measured, and with varying installations?

"Does this mean that you should not replace all of your batteries if you find one bad? Maybe, or maybe not." I'll agree with that!

But given all the variables, I still think that paralleling batteries gives the system opportunies for failure that simply do not exist when using batteries in series, or separate banks. Given optimization of the charging system (capacities and rates matched, etc.) I don't see anything to be GAINED by paralleling, except the chance to buy cheap 6V or 12V batteries from convenient sources.

Maybe we can get Practical Sailor to give us a stipend to do the lab testing, because without that...all we can say is "if, maybe, it depends, it might". Personally, I'd rather use a system that doesn't rely on those terms.<G>


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## sailandoar (Mar 20, 2006)

*Advantage of Parallel*

Without going back to read Nigel Calder....

The life of a cell is maximized by operating as nearly as possible to a fully charged state. When one increases (doubles) the ampacity of the bank by means of paralleling, for a given draw on the bank, one draws down the ampacity/energy pool LESS (as a % of total) and so operates closer to fully charged. I seem to recall that Nigel Calder in the book referanced in my post above outlines the case with specific examples and generates expected life extension of the battery bank which is much much more than negligable.

I will go back and read it ASAP, but it will certainly be sorted out in this thread before then.


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## dave.verry (Apr 5, 2006)

*Oh good, a challenge*

Hellosailor, I knew I could count on you!

All battery manufactures of lead-acid batteries recommend a constant voltage, or float voltage, method for maintaining the charge level of their lead-acid batteries. This is what the voltage regulator of any engine alternator does; maintain the output level at a constant voltage. Most battery manufactures also recommend limiting the charge current into the batteries to about 20% of the rated 8 hour amp hour capacity, for a 200 AH bank this equates to 40 amps. Charging the batteries with much more then this current would promote self heating, electrolysis and constitutes charging abuse. Generally this current is limited by reducing the voltage output once the charging has reached this current limit and the output is regulated by current until the float voltage level is reached. Some class of chargers will do a secondary charge level, termed boost, and allow the current to be maintained until a higher voltage is reached, on the order of 14.4 to 14.8 V, and then the float or maintenance level is resumed, somewhere between 13.3to 13.8V. Equalize charging should be done only when recommended by the battery manufacturer and using the manufactures specific procedure. This helps overcome plate sulfation and can reestablish correct specific gravity levels in all of the cells. Other special charge cycles are possible but not recommended unless you can monitor temperature, outgassing, current draw, etc.

Practically one limits (or increases) the size of the alternator to limit the current flow to the battery. In the case stated above, a 60A alternator would be about the maximum size suggested.

As the batteries charge, the electrochemical state within the battery changes and the equilibrium cell voltage rises. For a smaller capacity battery, the "bad" battery in our example, this equilibrium voltage will increase faster then the larger capacity battery for a given current. If the two are connected in parallel, the smaller capacity battery will require less current to charge to the same voltage level as the larger. This is the mechanism for the current being "shared" by the two batteries. The larger one will use twice the current as the smaller one to get to the same equilibrium voltage level.

Using our example of one "bad" battery and one "good" with a 40 amp charge current, the "bad' battery will charge at 13.3 amps, or 26% of the rated 8 hour capacity, and the good battery will charge at 26.6 amps, also 26% of the rated amp hour capacity. This is a little high but not catastrophic. We can also see in this mechanism that the paralleled battery system is "self healing", one battery will not be overcharged and one will not undercharge.

This is from both theoretical and empirical data. I have measured currents under similar conditions and the only time I did not get these results some other factor was present.

The discharge sharing is similar to the charging. The same reasoning applies.


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## btrayfors (Aug 25, 2006)

sailandoar,

Exactly right! Cycling a deep-cycle battery to only 30% discharge rather than 50% discharge on a regular basis can GREATLY extend the number of useful life cycles....as much as 3 times!

That's one of the principal reasons it makes good sense to parallel your batteries into a large battery bank. This larger bank will be drawn down less (smaller percentage) from full charge than would a smaller bank, given the same load.

Bill


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## hellosailor (Apr 11, 2006)

Sailandroar-
"The life of a cell is maximized by operating as nearly as possible to a fully charged state. When one...paralleling...and so operates closer to fully charged." 
A perfect example of fallacious logic! You are confusing the two issues, of discharge depth and parallelling. By using the correct cells, you can accomplish the same low-discharge-cycling WITHOUT paralleling. If you want to make a proper comparison, the comparison would be using a series of cells (a simple battery) versus a parallel array, of the same total capacity. In which case you'd find the cycling effects from the parallel setup to have no advantage at all.

Remember, if you have two batteries of "x" amp hours each...You can still cycle each one separately to 20-30-50% depth and still get the same total number of amp hours in and out of them--regardless of whether they are paralleled, or used sequentially. The depth-of-discharge argument holds no electrolyte here, if I may coin a pun.

Dave-
Regarding boost charges and such..."Other special charge cycles are possible but not recommended unless you can monitor temperature, outgassing, current draw, etc." All of which can't be accomplished properly with parallel batteries, since there is normally one charge voltage sensor, one battery temperature sensor, one &cetera and now, the question is which battery or bank do you hook it up to? And of course, that's all stuff that the smaller systems using automotive alternators just aren't going to have. 

"This is from both theoretical and empirical data. and the only time I did not get these results some other factor was present" And what other factors did you get? Factors related to the parallel construction, or factors that might, again, affect it differently?

Further:
Let's look at another issue that has been ignored. Cell failure. Whatever the odds of cell failure are (1:1000? 1:100,000?) for any individual cell, once you start using parallel batteries, you multiply those odds. If you have six cells, each with a 1:10,000 chance of failing, you now have a 6:10,000 chance of having a battery failure from a bad cell. But, if you parallel two such batteries...you've doubled the odds of having a failure, to 12:10,000. And one cell failure will still take down both batteries as it starts those current loops and failure modes that you say can be ignored--except in the case of a bad cell.
Keep the batteries separate, and use them sequentially, and your risk of failure is now effectively halved, since you've got redundant and separate banks. Use them in parallel...and all your eggs were in the one basket. I can't call that a good idea. Or do you dismiss that?


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## btrayfors (Aug 25, 2006)

I doubt that hellosailor has really had much experience cruising in the typical power-hungry sailboat with minimal battery capacity. If he had, he'd know that there aren't many sailors who are willing to keep their eyes glued on the voltmeter and, worse, get up at 3 or 4AM to switch from battery bank A to battery bank B so as to avoid a deeper than desired discharge.


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## hellosailor (Apr 11, 2006)

btrayfors, there's no need to wake up at 3AM to manage battery banks if you've been "managing" the power system in the first place. If I'm racing or cruising 24x7, we discuss power management, engine runs, and charging cycles in advance, with an eye towards switching banks optimally and not running the engine when an offwatch might prefer to sleep.

Criticizing the poster rather than contributing to the technical discussion, serves no purpose here. Dave and I can appreciate the concept of discussion without diverting into personal quests or attacks, you're welcome to join us on that level.

Yes, I have redesigned charging systems to match the components, and yes, I have managed energy budgets. Sometimes compromising "whats right" with "what we have to do" and "what we can afford".

But I've yet to hear any reason in favor of parallel batteries, any reason that makes them in any way advantageous to separate batteries, beyond "Well it's cheap and easy to get them that way." 

Cheap may be a good reason, but that doesn't make it optimum design practice unless "cheap" is your goal.


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## btrayfors (Aug 25, 2006)

Hey, lighten up!

I'll be happy to join you on any level you want, but only if you bother to read the posts. We've given you at least three perfectly good reasons why paralleling batteries for the house bank often makes sense, but you've chosen to ignore them.

B.


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## hellosailor (Apr 11, 2006)

B, I've read the posts. And as I've said, the "discharge depth" argument is fallacious, it has no bearing here. The only other firm and clear argument about parallel batteries has been that they are cheaply built using readily available parts, i.e. the batteries from WalMart or Sam's Club.

And I've conceded that cheap sometimes is good. But aside from "cheap"...there have been no other arguments made that indicate anything *certain* about any advantage of parallel batteries. 

At best, Dave has said that depending on various factors, in certain cricumstances, there may not be any disadvantage to their performance--and that's ignoring the extra failure modes that are possible.

I'm saying that any configuration which provides zero definite advantages, and doubles my chances of a failure, is a bad idea. Cheap batteries in parallel may be convenient--but I've yet to see any reason to think they're any cheaper, either. (Battery pricing being a very variable thing.)


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## btrayfors (Aug 25, 2006)

OK, let's try again...one more time.

A series/parallel setup of house batteries on a (larger) cruising sailboat may be advantageous because:

1. lower "depth of discharge" in real world usage (far from a fallacious argument);
2. lower acquisition cost;
3. smaller size...will fit where other options won't;
4. easier on your back, assuming it's you who have to schlep the batteries;
5. better availability worldwide...a key factor for cruising sailors;
6. faster charging, lower run times for engine and/or generator; and
7. endorsement by experienced sailor-engineers such as Calder and Verry.

Providing that the installation is a good one, the potential "downside risks" of paralleling batteries are few and, in my experience and reading, very remote. This is particularly true because the "paralleled battery installation" itself, with reasonable care and attention to battery maintenance, is likely to reduce the downside risk of, e.g., a shorted cell by reducing the degree of sulphation and by maintaining a higher state of battery health.

A final factor...then I'm done. You can never discount the human factor, and the ability of a tired or careless or ignorant sailor to inadvertently murder his batteries. By keeping the system simple: one house battery bank only, one voltage measurement only, one charging rate only, etc..... you significantly reduce the chance of this happening.

Bill


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## dave.verry (Apr 5, 2006)

BTW Hellosailor, I’ve been meaning to ask. A single point of failure for a connection will render the single bank as you advocate useless. A parrelled bank has a greater chance of being at lease partially usable, if at a lower capacity.

“I'd been told that's not the case, and that is the problem. They WILL equalize, yes. And then as each battery is different, and has a different rate of self-discharge (among other things caused by the physical changes and sulphation happening differently in each battery), as soon as they have equalized they begin to drift apart and the current loop between them starts to flow again. This process does not stop, it continues and it drags them down.”

Agreed, each battery will have a different open cell voltage and they will drift apart if they are not connected. This drift will cause a small current to flow until the equilibrium voltage of both batteries are the same. This will NOT however drag the batteries down. What is removed from one battery will be stored in the other. Discounting self discharge, the amount of charge will remain constant. The current will be very small and eventually stop. The total charge capacity of the battery bank will not change.

And just to clear it up, full discharge is whatever you define it as. The more and deeper you discharge your batteries the faster they will loose capacity. I routinely discharge batteries at work to 1.75V/cell (10.5V) to get full capacity out of the battery. (I say routinely, by that I mean during each discharge cycle, maybe ½ dozen times per year.) However, being cheep and not wanting to replace my batteries all that often, I also use the 50% discharge point as fully discharged, about 11.25V (1.88V/cell) under load.


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## sailandoar (Mar 20, 2006)

*????*



> The depth-of-discharge (DOD) argument holds no electrolyte here,


I am assuming that it is true that: "from an electro-chemical standpoint, a lead acid cell's life time is extended by maintaining it as close as possible to a fully charged state."

*If that is true* then why is the "DOD" not to be considered? I have read the posts (albiet quickly) and am not clear where that notion was disputed and laid to rest.


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## hellosailor (Apr 11, 2006)

Sailandroar-
"If that is true then why is the "DOD" not to be considered? "
Because, the DOD argument has nothing to do with how the batteries are ganged up. You can run your batteries to any DOD regardless of the series/parallel/alternating configuration. Bozo had red hair and big shoes, Bozo was a clown. Therefore everyone with red hair and big shoes is a clown? No, that's invalid logic. Same thing with the DOD argument. Invalid and fallacious since how deeply you cycle your batteries is a decision that can be made independent of how you configure (gang) them.

Bill-
OK, let me address why I disagree, one more time:
1. lower "depth of discharge" in real world usage (far from a fallacious argument);
A: Yes, fallacious and invalid. As above, you can cycle the batteries to any depth, regardless of configuration. That's not debatable, that's fact.
2. lower acquisition cost;
A: Still unproven. As I've said, yes, you an easily get cheap batteries from a bulk store. That doens't mean buying industrial batteries from an industrial source is any more expensive. 
3. smaller size...will fit where other options won't;
A: False again. Watt-hour for watt-hour, the battery bank size will not differ at all. And I can fit six 2.2v cells into a wide variety of spaces where your pair of 12V cells simply can't go. If anything, the advantage here is for 2.2v cells in series.
4. easier on your back, assuming it's you who have to schlep the batteries;
A: False again, same as #3. The 2.2v cells are available in a much wider range of sizes/weights.
5. better availability worldwide...a key factor for cruising sailors;
A: False again. Anywhere there is shipping and freight, there are industrial battery suppliers supplying the 2.2v cells for fork lifts. You may not see them on the main shopping street or by the upscale marinas--but they are there until you get way off in the boonies, at which point you are looking at "emergency repairs" not scheduled battery replacement. Since these batteries can be expected to have a 10-15 year life, there's no reason to worry about "Oh wow, I need 500AH of batteries RIGHT NOW!" for any purpose besides emergency repairs.
6. faster charging, lower run times for engine and/or generator;
A: Totally false. Charging speed will depend on the system chemistry (AGM versus wet) and the alternator capacity, and the battery capacity. Given the same battery capacity, chemistry, and a sufficient alternator output to match that, the charging time will be the same for either configuration.
and
7. endorsement by experienced sailor-engineers such as Calder and Verry.
A: Specious logic. Yes, it is pleasing to say that "someone said it is good" but we're looking for facts not opinions here. What Dave and Calder have said is that you can, under some circumstances, get away with it. They have not, to my knowledge, made any point that parallel battery installations are in any way BETTER. Again, aside from the fact that you can build them with WalMart batteries.

So out of your seven points we have:
1-Fallacious
2-Unproven
3,4,5,6-Simply false.
7-Specious

And your only point made is the one I've conceded all along, that you can certainly use cheap stuff from WalMart real easily in parallel banks.

Dave-
"A single point of failure for a connection will render the single bank as you advocate useless. A parrelled bank has a greater chance of being at lease partially usable, if at a lower capacity." I think you've got that backwards.<G>
A connection failure can be remedied with a wrench. A CELL FAILURE is the only thing that would take down the series battery bank. The odds of that same cell failure taking down your parallel bank are double or higher. IF you catch the failure in time, sure, you can split the bank and run on half of it. But you can also do that by using two banks. Unless your bank sizing and alternator are matched/mismatched in certain ranges, that's going to be a questionable point anyway. If you have a 500AH parallel battery bank, you'll need a 100AH continuous rated alternator to charge it in minimal time. Vary the alternator size, vary the bank size...and you may find that running two alternate banks provides a better answer--plus that redundancy.

Regarding the loop between the two batteries "The current will be very small and eventually stop. " This is the first and only time I've ever heard that. Have you measured this? With wet or AGM cells? What were the specifics?


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## dave.verry (Apr 5, 2006)

Sailandoar,

There are several wear-out mechanisms in a lead acid battery. The one the limited depth of discharge addresses is plate integrity. Basically, the discharge chemical reaction within a battery dissolves the lead off the anode and puts it into solution as lead sulphate. The cathode reaction dissolves the lead oxide of the cathode and puts it into solution as lead sulphate and water. Charging the battery drives the reaction in reverse and hypothetically replates the lead and lead oxide back on their respective plates. The deeper the discharge the more of the plates are put into solution and the plates become porous and spongy. As the material replates, it does not fill in all of the pores. There is also some tendency for the lead and lead-oxide to plate to impurities and precipitate to the bottom of the cell. Occasionally dendrils of lead can grow between the plates and short the cell. The precipitate of lead and lead oxide can build up in the bottom and short the plates as well. 

By limiting the depth of discharge the integrity of the plates remain higher. The amount of lead in solution is limited so the amount of precipitate and dendril growth is limited. 

This reaction is taking place all the time in a battery, whether or not the battery has a load connected. Charging the battery at the float voltage level forces the chemical reaction back as rapidly as it naturally occurs without a load (self discharge). This is why batteries don’t last forever, even with a float charge.


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## sailingdog (Mar 19, 2006)

One other point on depth of discharge. If batteries are deeply discharged, they stand a much higher chance of having the plates sulfate. The lead sulfate may recrystallize into a dense coarse-grained form which has high resistivity. This will often inhibits recharging or discharging and effectively reduces the capacity of the battery. By limiting the depth of discharge, you can help prevent plate sulfation. 

BTW, the plates will also sulfate if they are exposed to air....so checking the electrolyte levels is fairly important.


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## dave.verry (Apr 5, 2006)

Hellosailor,

“A connection failure can be remedied with a wrench. A CELL FAILURE is the only thing that would take down the series battery bank.”

Yes but while you found the wrench, and then located the problem your system will be without power. No lights, no autopilot, no GPS, no radio. Not a chance I would want to take. A cell failure in a series bank would reduce the bank voltage, not necessarily take the bank offline. Cell failures, when they occur, tend to fail short. The only open condition that I have observed during a cell failure is when the battery explodes.

“The odds of that same cell failure taking down your parallel bank are double or higher.”

Cell failures are cell failures. The rates of failure should be the same no matter which configuration you use. A cell failure in a parrelled bank will take that battery out, not the others. The equilibrium voltage would be reduced by one cell, about 2V and that one battery would stop discharging into the load. Now before you start, the remaining battery bank will not discharge into the battery with the bad cell, the cell voltages of the remaining cells of the bad battery will be higher then the discharge or load voltage of the remaining batteries.

Cell failures are rare compared with connection failures. Cell failures also happen gradually with the exception of mechanical damage or catastrophic explosions during overcharging. Connection failures, especially in a harsh environment like a marine environment are much more commen. Thermal expansion and loosening, corrosion, vibration all can cause failure of connections. One connection in series with your battery string that fails removes the string from the circuit. While one connection can bring down a paralleled bank, its more likely to remove part of the bank from the circuit rather then the entire string.

“Regarding the loop between the two batteries "The current will be very small and eventually stop. " This is the first and only time I've ever heard that. Have you measured this? With wet or AGM cells? What were the specifics?”

No, this is not tested, but is correct in theory. The theory is sound for all lead acid batteries. If you want I will test this during the weekend. I just happen to have two 92 AH batteries in my garage, one to recycle and one to install in the boat. The test will be simple and I have the equipment.


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## dave.verry (Apr 5, 2006)

Sailingdog,

Correct, as usual. By limiting the concentration of the lead sulphate in solution you limit the possibility of crystalization. Equalization may fource some of the crystals back into solution, but once those crystals form they provide seeds for more crystals to grow. By coating the plates they reduce the batteries capacity by effectively making the surface area of the plates smaller. 

This reduction in area will also accelerate plate wear-out by forcing the areas uncoated to provide more lead into solution, reducing plate integraty in that area. 

Sometimes I wonder that it works at all.


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## hellosailor (Apr 11, 2006)

Dave-
" Cell failures are rare compared with connection failures. " Hmmm...and you think there are going to be fewer connections with paralleled batteries than there would be with series cells? Only if you neglect the internal connections in that calculation. Good external conections are not hard to make, or ensure.

One also hopes that even with series cells, no prudent mariner is relying on ONE battery or bank. Even if their house bank is only one primary bank, surely there is a second starter battery which can be used to start the engine and provide power while a primary failure is being looked at.

“Regarding the loop between the two batteries "..."No, this is not tested, but is correct in theory."
I'd love to see you test that out, and see the results. Starting voltages, initial current flow between the two, and changes in that flow over time. Whether it goes down and stops, or goes down, levels, then reverses, etc. Since there is lag in each battery as the electrolyte locally saturates and then evens out again, the reaction will *not* be simple and uniform, each battery should be internally changing chemistry at different rates.


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## dave.verry (Apr 5, 2006)

OK, boys and girls, here’s what I propose for the experiment on current between paralleled batteries.

I have two AGM 92 AH batteries, one at 90% capacity and one at 55% capacity. I had finished a load test on them a couple of weeks ago so I know this is empirical data, not theoretical or estimated. Both batteries are Douglas Battery DGS12-100F and are eight years old.

Both batteries have spent the last several days on a charger and are fully charged as defined by the float current (both batteries where drawings less them 100 mA). They will sit disconnected from each other (and anything else) for the next 48 hours. At that point I will measure and record the open cell voltage of each battery.

I will then hard wire the negative terminals together using 8 AWG cable. At the start of the test I will connect a Fluke 8060A 5 ½ digit (three decimal places, at the 12 V range. It will measure 1/1000 of a volt) DMM in current mode (measuring in the 2A to 1 milliamp range) between both positive terminals. I will record the current every 15 seconds (using a camera as my automated equipment will be in my lab, not my garage). The data point time interval will be modified as the change in current dictates (no use recording a lot of points of the same level. If the slope is flat I will modify the time between photographs, but that time interval will be recorded.) The temperature of the batteries will also be recorded.

Results will be graphically represented in this thread when completed.

This should answer any questions about current flow between paralleled batteries.

Comments? Suggestions? Hellosailor, you asked for this. Now is the time for comment.


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## cardiacpaul (Jun 20, 2006)

I'm waiting with baited breath for the results. 
(The Cuban says "no more sardines for breakfast for you")


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## camaraderie (May 22, 2002)

Go for it Dave!


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## sailingdog (Mar 19, 2006)

Looking forward to your results.


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## hellosailor (Apr 11, 2006)

Dave, that sounds very logical. Camera, eh? Nice way to kludge a data recorder!<G> I'd also be interested to see what voltage they are at 2-4 days after the start, in comparison to the starting voltage.

I'm unsure if the test is more--or less--reflective of real world conditions by letting them settle for 48 hours after charging. In the real world, they'd be connected during and after charging, so perhaps the comparison would change if they were tested that way as well?

I've got vague memories that one of the equations I never remember (Kirchoff's? Thevinen's?) can be used to model two batteries hooked up just to each other but then again that's theories, and I'm almost afraid to ask how the theoretical internal resistance of a battery varies against reality in the lab.<G>

Wait, let me call my agent, I think I can get Fox and Geraldo to supply armed Pinkerton's to guard it over the weekend, and unveil the results on national TV come Monday. There could be some heavy sponsorhip money in this.<G>


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## dave.verry (Apr 5, 2006)

I figured I would do worse case at initial. If they were connected together they would both start at the same voltage, not much current flow with no potential difference. 

I will run the experiment for most of the week. However, I will not be able to get data on Monday and Tuesday, will be out of town for business. However, this will give them more time to settle. I anticipate the current flow will be at a minimum by the end of Saturday, but we’ll see.


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## hellosailor (Apr 11, 2006)

Dave-
I look forward to the results, no matter what they are.

CardiacPaul-
If you didn't want to further hijack the other thread, why didn't you leave it at "See my comments in..." instead of hijacking it further? I can see why you seem so upset at everything I suggest. Every time I give one or more examples of what's possible, you seize one and rail against it as dogma. There's a big difference between claiming dogma, and suggesting options. I'm only suggesting options.

Your scenarios about Rolls batteries and warranty claims in foreign ports are way too creative for me. If I needed a battery in some foreign port, and wanted it via warranty, I'd look first for local agents. Rolls might have them. If they didn't, I'd ask them to ship the new cell DRY to the nearest US port via Parcel Post or similar economy means, not demand they needlessly ship them as overnight hazmat. Yes, battery acid can be obtained in most places where you find cars and telephones.
But I don't think I'd bother with all that fuss in the first place. I suspect that in any Caribbean port, you'll find there are port industries and fork lifts, and along with them, a convenient local way to buy the one cell you need to replace. You're way too zealous about seizing on one idea and ignoring the options.
Like, insisting parallel batteries must be better simply because a lot of folks were taught to use them. Convenient? Maybe. Cheap? Maybe. But that's like saying automotive alternators are the best ones to use on boats--because so many boatbuilders equip all their new vessels with them.

So let's try to make this clear: If you have a twelve volt battery on your boat, any kind of 12v battery on your boat, you ALREADY HAVE ONE SET OF CELLS IN SERIES. Anything you add to that, adds potential problems. Instead of adding the potential problems, if you simply choose the original six cells for adequate sizing and other constraints, you have the simplest and most reliable 12v battery. 

Maybe not the cheapest, certainly not the easiest to obtain. But simplest, most reliable, and with a little perseverance about looking for the options, quite possible the best range of size, fit, weight, installation configurations, and other features.

You can't make one 12V battery, unless you make one SERIES CELL BANK. If you reject the validity of series cells...you're stuck at 2.2v for your ship's power standard. (Or 1.5, or 4.3, etc. whatever chemistry you choose.)


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## btrayfors (Aug 25, 2006)

Uh huh! Nice analysis, Hello Sailor! At least as far as it goes...

Now, let's get real world. Quite apart from cost (2-3 times at least) and size (where the hell do I put these tall batteries) and maintenance (12 cable connections to inspect and maintain instead of 2), what happens when you lose a cell during a passage?

You're two days out of LA headed for Hawaii, and one of the cells on your ONLY 12V house battery develops a short. This might be because of bouncing around shook something loose inside, sulphate buildup in the bottom, plate corrosion, etc., etc. In any case, instead of 12.6VDC you now have a 10.4VDC house battery bank. Not enough to run your SSB, or other appliance which needs at least 11-12VDC to operate.

You're many days from shore. You're many more days from obtaining a replacement for the bad cell. Oh, bother!

With a paralleled house battery bank what do you do? You just flip the battery switch from "ALL" to "1" or "2" to isolate the bad battery bank, and carry on as before.

Do you think the sailor with the paralleled battery bank gives a damn that he/she may be losing a tiny percentage of efficiency by paralleling the house battery banks???

Bill
S/V Born Free


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## sailingdog (Mar 19, 2006)

Btrayfors-

Actually, you would probably just yank the wires from the bad battery...as you want to have separate house and starting banks, unless your engine is small enough to start by hand, which is not the case for most in-board diesels.

For the most part, I'm not talking about paralleling the house and starting banks, but having several 12V batteries in parallel to create a larger 12V bank.

Your point is still quite valid though. If a single 2V cell in the large battery banks that hellosailor is advocating, you're basically screwed. Apparently, that thought has not occurred to him.

*Personally, I'd rather have a half or two-thirds my battery capacity at the right voltage, than almost 83% of my capacity at too low a voltage to run anything safely.*


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## hellosailor (Apr 11, 2006)

Bill-
"let's get real world."
By all means. Just bear in mind that every time we go over one set of criteria or options--that's only one set, not dogma, not one size fits all.

"Quite apart from cost (2-3 times at least)"
Cost based on what? If you haven't shopped for industrial cells locally, you don't know the costs. I can buy a 12V 80AH deep cycle from WallyWorld for moaybe $55. Or $85 from the battery store. Or $129.95 from the auto parts store. And they're all rated "the same". Prices vary widely and the only prices that anyone has posted for cells here--are the ones that I said represent the HIGHEST END. Now consider these are industrial cells (like Rolls uses) and the users expect 10-15 years of life out of them. Still want to compare them, at any price, to the WallyWorld battery that will last 4-5 years tops?

"and size (where the hell do I put these tall batteries) "
Again, look at the options, don't get stuck on what I said were first hits from the web. These are not tall batteries or tall cells. You can buy them tall--OR SHORT. Remember, Rolls is using these cells in conventional boxes that are sold in conventional sizes and shapes. This is an OPTION not a RESTRICTION.

"and maintenance (12 cable connections to inspect and maintain instead of 2), what happens when you lose a cell during a passage?"
Maintenance? What maintenance? My "cousin" drove a main battle tank and his daily maintenance including using a mirror on a stick to observe electrolyte levels in the battery cells, because they were located where heads couldn't get. You don't need cable connections for these cells. Depending on the options you have chosen, you can bolt them directly together (again, as Rolls does internally) or bolt them with buss bars, or use cables. I'd use cables only if they were the right solution to get more cells into a more restricted space. And if you know how to use bolts, they don't just let go. Like any critical bolt, you need to at least come close to the right torque setting. You'll see Dave's pictures above of battery labels actually tell you what torque to use on them. And you use nylocks, or thread lock, or locking (double) bolts, as apropriate. Yes, it is possible to use bolts reliably.

But more important, how do you think the six cells in each of your parallel batteries were assembled? Ooops, no bolts, they are welded and sealed, no access possible. If a weld was bad in that cheap battery--you lost it. So maybe bolts aren't so bad after all...somehow, they hold most of the boat together.<G>

"You're two days out of LA headed for Hawaii, and one of the cells on your ONLY 12V house battery develops a short. "
OK, so what? If that happens in a "12v" battery, you're screwed, you've got no battery. If it happens in your ONLY battery, you're still screwed, no matter what kind it is. The debate over series/parallel should not be confused with the debate over whether to put all your eggs in one battery. I'd suggest that a second battery, whether it was an SLI battery or a second house bank, would still be prudent. But again--that's a totally separate issue!

"Do you think the sailor with the paralleled battery bank gives a damn that he/she may be losing a tiny percentage of efficiency by paralleling the house battery banks???"

Nope. Most boat owners (and remember, most are not global cruisers and most have boats under, what is it, 32' OAL?) are also satisfied with automotive charging systems and replacing their batteries every 3-4 years.

And those are also the folks who run "A+B" all the time, and call TOW_BOATS_R_US around 4PM Saturday when the stereo has killed both batteries.<G>

Does that mean they're doing things the best possible way? No. But that's a pretty good thing, if you run a tow boat for a living.<G>


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## sailandoar (Mar 20, 2006)

*Pictures! / Stats! / Sources!*

Hellosailor,
Of all the posters in this thread, you seem to be the most familiar with single cells. While I realize that every battery is just 'bank in a box' it is still a sort of wierd and foriegn notion to most folks to build your own battery. Can you provide links to web pages that show pics and stats and prices of individual cells?

The only time I have been 'up close and personal' with individual cells is in the emergeny power room of a Nuc. Plant where the cells were in glass containers about a foot square and 3 feet high. Other than that I remember seeing pictures of batteries being repaired back when the cells were set in tar in the box and one used a hot knife to melt out the bad cell and insert a new one. Anyway I understand that SHORT cells must be available because that is what is used to make Rolls 8D's etc etc, but how to find them. I looked around a bit on google and found US Battery and Trojan but they only had 6V, 8V, 12V batteries that I saw. I did find 4V on the surrette site but the shortest was 16" tall.

May personal preferance so far is to have two twelve volt banks that can be paralleled if desired and I would leave them paralleled most of the time. I would prefer to build my own bank out of 2V or 4V cells.


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## hellosailor (Apr 11, 2006)

Sailandroar-
"it is still a sort of wierd and foriegn notion to most folks to build your own battery.' Consider that most of us, myself included, talk about buying AA batteries or D batteries for a flashlight. Ain't no such thing, a "D" cell is a cell. A 9-volt battery is indeed a battery of six smaller cells. A watch battery...is again a cell, not a battery.<G> So yes, we all have some foreign notions. After all, the grade schools don't teach you 'A boat is a hole in the water that you throw money into' either, do they?<G>

I haven't seen much about them on the web. Dynabattery.com has a list of their standard products. East Penn/Deka doesn't seem to have any list, or even indicate they make them on their web site. Considering that lead cells are, after all, LEAD and shipping costs can be killer on them, I think it is more effective to look in your local Yellow Pages for a local distributer. They're usually located in some grimy industrial corner but a few phone calls should track them down. And a call to any major manufacturer (JCI, Interstate, East Penn, etc.) should be able to wring some kind of industrial contact number out of them. Just tell 'em you have a fork lift, or a nuclear reactor<G>, and you need a local source of battery cells.

And of course, if you're under 36' and you're not running a reefer and AC, a pair of Group31's might be all you need. I still like the concept of separate and redundant supplies, unless there's a good reason to match a big charger to "one big bank". And even then, I'd want an isolated "spare". Maybe an SLI for the starter, or a separate deep cycle forward on the winch, or even a relatively small 12v17Ah AGM battery, 1/3 the size of a shoebox, which makes a good emergency starter even for a small diesel engine. (I'm sure the current draw violates proper usage, but "emergency" is the trump card.)


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## dave.verry (Apr 5, 2006)

*Preventive Maintenance*

Oh, oh, Hellosailor, No thread lock. It does not guarantee a good connection or a connection that won't corrode. As I stated before, preventive maintenance must be performed on battery systems, series or parallel. One of the most important chores is inspection of the connections for tightness and corrosion. There are literally hundreds of connections in a typical battery installation for telecom and each one must be checked to insure good contact, no corrosion and correct torque. You can be sure that during maintenance of that tank that one of the routine chores was to clean and tighten those connections.

These connections "settle" during thermal cycling and vibration and you will be able to tighten them even though they have not backed off or turned.

All connections, bus bar or crimped, must be regularly checked.

Connections on a boat should be inspected at least once a month. I actually just got back from a sail this afternoon and installing additional batteries to my bank and even though I regularly inspect my connections, I found two loose and one starting to corrode.

Preventive maintenance MUST include inspection and retorquing battery connections, no excuses.


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## btrayfors (Aug 25, 2006)

"I still like the concept of separate and redundant supplies, unless there's a good reason to match a big charger to "one big bank". And even then, I'd want an isolated "spare". Maybe an SLI for the starter, or a separate deep cycle forward on the winch, or even..."

Well, now you're talking! This is exactly what we "parallelists" have been talking about all along, although it seems like we're trying to nail jelly to the wall to get this concept through.

- Yes, two or more house banks to provide redundancy. 
- Yes, combine them for efficient charging with a large alternator and smart marine regulator. 
- Yes, combine them for convenience and for "shallow cycling" rather than "deep cycling", without having to switch between battery banks. 
- Yes, a completely separate battery for the engine. 
- Yes, redundancy....I have that bank forward you mentioned (two golf-carts) dedicated to the windless. There's also the separate engine battery.

Where we differ is with regard to your conviction that a reasonably-priced battery bank -- or two of them for redundancy -- can on most modest cruising boats be built up from readily available, not-too-costly, and not too tall 2V cells.

Let's move out of the general to the specific. Could you please recommend -- with specificity -- brands, sizes, and costs for a 12V house bank (say, 450-700AH at the 20-hour rate) which will fit into a battery space with a height of not more than 11-12"? 

No waffling about look at local dealers, search the Internet, etc. Let's have your specific recommendations.

Bill


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## cardiacpaul (Jun 20, 2006)

I'm done with the theortical BS that hellosailor puts out, gimme real world prices and shipping,(hasn't been done) tell me how you did it on yours, post a jpg of them on your boat, or off with your head! (ok, just kidding about the last part) 

I don't mind "options" hey, throwing a 454 big block in the stern is an option too, just not very practical. or real world

first, rolls doesn't ship just the cells, they want the whole battery and will ship you a whole battery... have you EVER SHIPPED 200 lbs via Parcel Post? 
If I need a new battery, I don't want to spend a week or more in port waiting for UPS or FED EX Ground, and just for grins, the bahamas aren't all that out of the way for many sailors here. AND I've seen enough of the grimy industrial portions of the bahamas to last a lifetime, thanks. I'm not saying that parallel must be better, but, damn, buddy, the whacked out "options" you give are... whacked out.

I did notice that you rebutted none of the points I've made, nor, told us about the fashionable cells in your boat.

and while I'm at at it,just gander at the number of distributors in the bahamas...from rolls...its DC Battery in Miami. sorry.

Now, suppose you come up with local distribs and prices for those 2 volt cells for me in oh, say the Bahamas? or hell, LA an Houston, Baltimore, and Bangor and, Miami. Surf theory boy, surf!


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## sailandoar (Mar 20, 2006)

*A little lite reading !*

A little light reading. Don't have any ties to these folks but the info seems resonabley complete, clear and good. 
For a good compromise on size and weight and until the "buy a small 2Volt cell issue" gets sorted out the 6V golf cart batteries seem a good choice. I have L-16's and a place to put them, however at 124lbs and 16+" tall but I would not wish them on anyone that did not require very high capacity. I will keep looking for 2V cells but so far I have found 4V cells (24" tall) at several 'off the grid' places on the web. Any of those folks that specialized in 'OFF THE GRID' solar/wind etc are a wealth of info on things battery.

_*Excerpted from:*_
http://www.windsun.com/Batteries/Battery_FAQ.htm#Major Battery Types

*Lifespan of Batteries*

The lifespan of a battery will vary considerably with how it is used, how it is maintained and charged, temperature, and other factors. In extreme cases, it can vary to extremes - we have seen L-16's killed in less than a year by severe overcharging, and we have a large set of surplus telephone batteries that sees only occasional (5-10 times per year) heavy service that are now over 25 years old. We have seen gelled cells destroyed in one day when overcharged with a large automotive charger. We have seen golf cart batteries destroyed without ever being used in less than a year because they were left sitting in a hot garage without being charged. Even the so-called "dry charged" (where you add acid when you need them) have a shelf life of at most 18 months, as they are not totally dry (actually, a few are, but hard to find, the vast majority are shipped with damp plates).

These are some general (minimum - maximum) typical expectations for batteries if used in deep cycle service:
Starting: 3-12 months
Marine: 1-6 years
Golf cart: 2-6 years
AGM deep cycle: 4-7 years
Gelled deep cycle: 2-5 years
Deep cycle (L-16 type etc): 4-8 years
Rolls-Surrette premium deep cycle: 7-15 years
Industrial deep cycle (Crown and Rolls 4KS series): 10-20+ years
Telephone (float): 1-20 years. These are usually special purpose "float service", but often appear on the surplus market as "deep cycle". They can vary considerably, depending on age, usage, care, and type.
NiFe (alkaline): 3-25 years
NiCad: 1-20 years

*Battery Size Codes*

Batteries come in all different sizes. Many have "group" sizes, which is based upon the physical size and terminal placement. It is NOT a measure of battery capacity. Typical BCI codes are group U1, 24, 27, and 31. Industrial batteries are usually designated by a part number such as "FS" for floor sweeper, or "GC" for golf cart. Many batteries follow no particular code, and are just manufacturers part numbers. Other standard size codes are 4D & 8D, large industrial batteries, commonly used in solar electric systems.

Some common battery size codes used are: (ratings are approximate)

U1..................34 to 40 Amp hours......12 volts
Group 24..........70-85 Amp hours.........12 volts
Group 27..........85-105 Amp hours.......12 volts
Group 31..........95-125 Amp hours.......12 volts
4-D................180-215 Amp hours.......12 volts
8-D................225-255 Amp hours.......12 volts
Golf cart & T-105.......180 to 220 Amp hours.........6 volts
L-16.........................340 to 415 Amp hours........6 volts

*Cycles vs Life*

A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to 20%, and then back to 100%. However, there are often ratings for other depth of discharge cycles, the most common ones are 10%, 20%, and 50%. You have to be careful when looking at ratings that list how many cycles a battery is rated for unless it also states how far down it is being discharged. For example, one of the widely advertised telephone type (float service) batteries have been advertised as having a 20-year life. If you look at the fine print, it has that rating only at 5% DOD - it is much less when used in an application where they are cycled deeper on a regular basis. Those same batteries are rated at less than 5 years if cycled to 50%. For example, most golf cart batteries are rated for about 550 cycles to 50% discharge - which equates to about 2 years.

How depth of discharge affects cycle life on batteriesBattery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis. This does NOT mean you cannot go to 80% once in a while. It's just that when designing a system when you have some idea of the loads, you should figure on an average DOD of around 50% for the best storage vs cost factor. Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film.









The graph above shows how lifespan is affected by depth of discharge. The chart is for a Concorde Lifeline battery, but all lead-acid batteries will be similar in the shape of the curve, although the number of cycles will vary.

*Amp-Hour Capacity*

All deep cycle batteries are rated in amp-hours. An amp-hour is one amp for one hour, or 10 amps for 1/10 of an hour and so forth. It is amps x hours. If you have something that pulls 20 amps, and you use it for 20 minutes, then the amp-hours used would be 20 (amps) x .333 (hours), or 6.67 AH. The accepted AH rating time period for batteries used in solar electric and backup power systems (and for nearly all deep cycle batteries) is the "20 hour rate". This means that it is discharged down to 10.5 volts over a 20 hour period while the total actual amp-hours it supplies is measured. Sometimes ratings at the 6 hour rate and 100 hour rate are also given for comparison and for different applications. The 6-hour rate is often used for industrial batteries, as that is a typical daily duty cycle. Sometimes the 100 hour rate is given just to make the battery look better than it really is, but it is also useful for figuring battery capacity for long-term backup amp-hour requirements.
Why amp-hours are specified at a particular rate:

Because of something called the Peukert Effect. The Peukert value is directly related to the internal resistance of the battery. The higher the internal resistance, the higher the losses while charging and discharging, especially at higher currents. This means that the faster a battery is used (discharged), the LOWER the AH capacity. Conversely, if it is drained slower, the AH capacity is higher. This is important because some folks have chosen to rate their batteries at the 100 hour rate - which makes them look a lot better than they really are. Here are some typical battery capacities from the manufacturers data sheets:
Battery Type.............100 hour rate..........20 hour rate.......8 
Trojan T-105................250 AH...................225 AH.........n/a
US Battery 2200..............n/a......................225 AH........181 AH
Concorde PVX-6220.......255 AH...................221 AH.........183 AH
Surrette S-460 (L-16)....429 AH...................344 AH.........282 AH
Trojan L-16..................400 AH...................360 AH...........n/a
Surrette CS-25-PS........974 AH...................779 AH..........639 AH

*State of Charge*

Here are no-load typical voltages vs state of charge

(figured at 10.5 volts = fully discharged, and 77 degrees F). Voltages are for a 12 volt battery system. For 24 volt systems multiply by 2, for 48 volt system, multiply by 4. VPC is the volts per individual cell - if you measure more than a .2 volt difference between each cell, you need to equalize, or your batteries are going bad, or they may be sulfated. These voltages are for batteries that have been at rest for 3 hours or more. Batteries that are being charged will be higher - the voltages while under charge will not tell you anything, you have to let the battery sit for a while. For longest life, batteries should stay in the green zone. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life considerably. It is important to realize that voltage measurements are only approximate. The best determination is to measure the specific gravity, but in many batteries this is difficult or impossible. Note the large voltage drop in the last 10%.
State of Charge........12 Volt battery............Volts per Cell
100%.............................12.7.........................2.12
90%...............................12.5.........................2.08
80%...............................12.42.......................2.07
70%...............................12.32.......................2.05
60%...............................12.20.......................2.03
50%...............................12.06.......................2.01
40%...............................11.9.........................1.98
30%...............................11.75.......................1.96
20%...............................11.58.......................1.93
10%...............................11.31.......................1.89
0...................................10.5........................ 1.75

********* end of excerpt ********
http://www.windsun.com/Batteries/Battery_FAQ.htm#Major Battery Types
*****************************


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## hellosailor (Apr 11, 2006)

Cardiac? "gimme". 
Sorry, Santa says you'll have to get them yourself. I'm not going to find your local yellow pages and chase down your local industrial suppliers. 
What I'm sailing on right now requires a lot less power, I go A/B and alternate two Group31's and that's enough for my modest needs. No reefer, no stereo, and no big budget to be invested. Now, seize on that and proclaim that means I'm unable to do the math and planning for a larger installation, by all means. That would be like saying the design team for the International Space Station are unqualified, because they've never been in orbit before.<G>

Sailandroar:
"6V golf cart batteries seem a good choice." Yes. And yes again, height can be a problem. AGM would allow you to lay those down--but AGM is a whole other set of considerations and price increases too. two good points: No acid leaks and 25% faster charging rates. Two bad points: Can't replace electrolyte, and a 20-40% price difference. Ooopsie.<G>

From your web quote:
"Deep cycle (L-16 type etc): 4-8 years
Rolls-Surrette premium deep cycle: 7-15 years
Industrial deep cycle (Crown and Rolls 4KS series): 10-20+ years"

And that's something else I'm afraid totally eludes CardiacPaul. The 2V cells are industrial cells, with a typical lifetime 2x-3x longer than the "marine" 12V batteries from WallyWorld. If, IF, they cost a bit more...that's still a different price for different goods. But some folks think all batteries are the same. No skin off my back.

Windsun's range of useful charge from 10.5 to 12.7 volts is a bit generous, the old scale from Practical Sailor that I have on my hydrometer case runs from 12.6 to 11.6 and from experience I can say that at 11.8 volts they're still useful--soetimes--but in my book, well overdue for replacement.<G>

DAVE!
I did some homework, check me out on this, will you? Using Thevenin's equation the current flowing between two batteries should be equal to the voltage difference between the two batteries and the total of the two battery resistances, yes?

So let's rashly and randomly pick two of the common "one man can easily lug these" Group27 batteries, rated at 12.6V 88AH nominal capacity, off the same production group with a difference of 0.1V in their "fully charged brand new" state. The mfr's data I've seen claim these can put out 3300A into a dead short and that's close to the internal resistance claimed of roughly 0.004 ohms, cited at 60Hz not at DC.

With some rough numbers, that tells me the current flow between the two batteries will be 12.5 amps when they are initially connected. ( 0.1v divided by 0.008 ohms) Once the battery voltages equalize--which should be, what, five or ten minutes? that current flow should drop to near zero. So the 12-amp flow is not a concern IF it only lasts once for a few minutes.

And if the two batteries were finely matched and only differed by 0.01 volts, the flow would be a piffling 1.25 amps initially.

Do those numbers sound right (ballpark) to you? Indicating that the internal impedance of the battery, and the match of battery voltages, will be a major factor for those first minutes?

But then, what happens in the real world, as both batteries self-discharge and the acid in them equalizes charge, at different rates?

I guess that's what your experiment will tell us, I'm not sure how to model things past that.


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## dave.verry (Apr 5, 2006)

Hellosailor,

Sounds correct, however as you mentioned, things sometimes happen differently in the real world.

BTW, that current represents energy going from one battery to the other. The total energy of the system (discounting self discharge, which is really a chemical reaction, and resistive heating which will be negligible) remains constant. So even with the current flow you won’t be loosing any capacity. Once you put a load on the system BOTH batteries will discharge into the load.


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## cardiacpaul (Jun 20, 2006)

You said...

"Cardiac? "gimme". 
Sorry, Santa says you'll have to get them yourself. I'm not going to find your local yellow pages and chase down your local industrial suppliers. 
What I'm sailing on right now requires a lot less power, I go A/B and alternate two Group31's and that's enough for my modest needs. No reefer, no stereo, and no big budget to be invested. Now, seize on that and proclaim that means I'm unable to do the math and planning for a larger installation, by all means. That would be like saying the design team for the International Space Station are unqualified, because they've never been in orbit before.<G>"

I have a double major, in CompSci and Math, so I'm more than qualified to separate theory from practice, and practice from BS.
You provided no sources (not even from your own town) no real world, just theory. your "cred" just tanked. Sorry.

BTW, no offense to anyone, but do you know the difference between a Theory and a hunch? ... an MBA. I'm going sailing.


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## hellosailor (Apr 11, 2006)

Dave-
"BTW, that current represents energy going from one battery to the other. "
Not really. It is modelled on a resistive load and resistive loss, the weaker battery might as well be a light bulb as I understand it.

But I'm surprised no one put two and two together. Run the model, and now run it with the case of one bad cell in one battery. What's a bad cell? Well, if it fails open...that battery is out of the circuit. If it fails by shorting (an internal short, which appears to be the most common reason cells fail)...Ah, Dave, I would have thought you'd see that one by now. 
Battery with all good cells: 12.6V. Battery with one bad (shorted) cell, 10.4V. I figure that one shorted cell, which normally would "just" weaken one battery, would now cause a 275 Amp current to flow down from the good battery. 
That's gonna hurt, no matter where you think the current goes. Especially if the batteries don't each have a primary fuse right on them. And in two paralleled batteries...that's what will happen if one cell in either of them shorts out. A potential problem--and fire--that the series cells wouldn't have.


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## dave.verry (Apr 5, 2006)

Hellosailor,

“"BTW, that current represents energy going from one battery to the other. "
Not really. It is modelled on a resistive load and resistive loss, the weaker battery might as well be a light bulb as I understand it.”

Then why does a battery charge? If it only represented a resistive load (as your comment indicates) then there would be no such thing as charging.

Your model is incomplete. Yes there is resistive component, but much more then that is the capacitive element of the model. A battery is roughly modeled by a capacitor of 1 farad per amp hour in series with the resistive component. The capacitor prevents the battery from behaving in the way you describe. The resistive element dissipates power while the current is flowing, but if you calculate the wattage dissipated using W=(I*I)*R, you will discover that the wattage lost is small, very small at the currents between batteries when paralleled. The capacitive element stores the charge transfer from one battery to the other and prevents the battery for behaving like you describe.

Just as a preliminary report, the batteries under test drew 300 mA when first connected and now after 24 hours is drawing 5 mA. There has been no current reversal, just a steady geometric regression to zero. I will continue the experiment for the next half week.

I now understand why you don't think paralleling batteries work. Reexamine your objections based on this new model. Yes this model is correct.


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## btrayfors (Aug 25, 2006)

Dave,

Good job! And very useful info. Thanks for your efforts on this question.

Bill
S/V Born Free


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## capn_dave (Feb 17, 2000)

*Right on Dave*

I have been waiting for you to fire the capacitor effect of batteries. In fact they make great filters.

Seems you are holding your own so I'll just lurk and enjoy the show. 
This is almost like writing my paper for my Masters in electronics.

Good job and fair winds

Cap'n Dave


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## dave.verry (Apr 5, 2006)

*umm...*

Cap'n,

Just trying to understand his objections, I made the mistake of not defining basic models and getting everyone on the same page, and I know better.

This has been enjoyable and allowed me to reorganize concepts that I have been using for years and haven't had to formally present for a decade. I hope everyones getting the understanding they need for their own system. I have tried to keep it from getting too technical so everyone can follow. Let me know if I stray.


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## Rockter (Sep 11, 2006)

I paralleled 3 of 100 Ahr Interstate "Deep Cycle" batteries in Houston in 1992, and they lasted me 10 years. That wasn't too bad, really, and they were not expensive. I have a dedicated starting battery also.

I bought a great big single unit 275 Ahr beast in 2002, and it cost me a whopping £325.... about $550. It had kevlar construction, or something marvellous like that.

I have had it 4 years and it refuses to lose charge (noticeably) if you leave it for months.

Long may it continue.

On the earlier threads.... I never would parallel a good battery with a known under-performer.

The chap was right about paralleling not making a difference to discharge.... it really does not. If you have 300 Ahr available and draw 1/3 of it, that is eaxctly the same as drawing the parallel bank down by the same amount, no matter how many batteries you have in parallel.....assuming both scenarios have the same capacity.

I only run the fridge when the engine is running, so tha battery is rarely discharged more than about 24 Ahr. Most of the sailing we do no is just in a Scottish canal. It must be very different for the long haul merchants in the tropics though.


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## dave.verry (Apr 5, 2006)

*Experiment Results*

I have completed the experiment as far as I am inclined to at the moment. I thought that you would like the results. I will be unavailable for the next week and a half, not sailing, but my daughters wedding. I will respond to any questions on my return, or just let this thread die it there is nothing further. I hope this has been useful to some.

Both batteries were fully charged over several days (< 0.1 amp draw @float voltage) and left dormant, open circuit without charge or load, for 48 hours. The open cell voltage was then recorded. The negative terminals connected together using an 8 AWG cable. The positive terminals where connected together using a Fluke 8060A 5 ½ digit (three decimal places, at the 12 V range. It will measure 1/1000 of a volt) DMM in current mode measuring in the 2A to 1 milliamp range between both positive terminals. I recorded the current every 15 seconds for the first hour and a half and then at increasing intervals. The temperature of the batteries was also recorded.

Battery #1 had an open circuit voltage of 12.628V and battery #2 an open circuit voltage of 12.681V before the start of the test. At time zero, at initial connection the current between the batteries was 30.35 mA (I had stated before that the current was 300 mA and I apologize, I was reporting from memory, never a good idea) [1 mA = 0.001 Amps] The scale on the DMM was reduced to 200 mA.

The results are shown in the attached graph. Additional observations are that there was a current reversal where the temperature and other fluctuations within the batteries caused changes in the open cell voltage. The current measured at this point was less then 1 mA and stayed less then 1 mA. (Just a quick calculation, IF the 1mA drain was into a resistive load and NOT into the capacitive load of the battery, i.e. the 1 mA was lost and not recovered, that current drain would represent a battery life in excess of 15 years with no recharging [discounting self discharge]. What this means is that this drain is less then the self discharge rate). The battery temperature remained stable at between 77 deg F and 80 deg F, fluctuating with the ambient temperature. The final voltage of the batteries at the end of the test were battery #1 = 12.629V and battery #2 = 12.681V. Battery #1 is the one at 90% capacity and battery #2 the one at 55% capacity. Yes, the open cell voltage for the bad battery was higher then the good battery.


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## sailingdog (Mar 19, 2006)

dave- 

Just curious, why the huge gap in data points?


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## dave.verry (Apr 5, 2006)

*Gap*

Business trip. Very hard to take points when your not there.


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## btrayfors (Aug 25, 2006)

Dave,

Thanks very much for your work on this. Very helpful information.

Would you care to go any further, e.g., in drawing any conclusions about paralleling batteries?

Bill


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## dave.verry (Apr 5, 2006)

*Conclusion*

Sure, it's what I stated up front.

Paralleling batteries is a practical, safe and reliable way to increase battery system capacity on a boat. Paralleling is naturally redundant and a bad battery will not take your battery bank down, nor will it ruin the remaining batteries in the group. A bad battery will reduce the capacity of the total system only by its own loss of reserve capacity. Paralleled batteries will NOT self discharge into each other and the total capacity of the system is the sum of the individual capacities of the individual batteries.

A single battery bank will not increase the total reserve time over two individual banks, assuming they are both of equal total capacity, but it will simplify charging and help eliminate over-discharging one bank by human error, i.e. forgetting to switch banks.

Parrelled banks will have a higher reliability then a single large bank as not only the batteries are parrelled but so are some of the connections (that's not to say that a single connection failure cannot bring down both parrelled and series battery banks, but there are a fewer number of connections in a parrelled bank that can bring down the entire system.)

In short, paralleling batteries works, and has some advantages over series individual large cells.


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## sailingdog (Mar 19, 2006)

dave.verry said:


> Business trip. Very hard to take points when your not there.


I figured it was something like that, but good to have verification. If you were available, I don't doubt you would have taken the readings per the schedule.


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## cardiacpaul (Jun 20, 2006)

thank you!


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## hellosailor (Apr 11, 2006)

Very interesting results, Dave. I'm not sure I've absorbed them all and had time to think them over, but something bothers me in the data. You say "The final voltage of the batteries at the end of the test were battery #1 = 12.629V and battery #2 = 12.681V. Battery #1 is the one at 90% capacity and battery #2 the one at 55% capacity."

A 35% difference in capacity, measured as how?? And the battery that you show as lower capacity has MORE voltage??? Isn't that, ah, physically impossible? Doesn't the voltage available decrease as capacity decreases?

I'm not sure we're seeing everything here, this 'bump' indicates something is out of sorts.

But more important--enjoy that wedding. A ten day long wedding party, wow, that's got to be fun.<G>


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## dave.verry (Apr 5, 2006)

*Test Battery Capacity.*

Each battery was in a single 48V string that was in service in my lab for the last 5 years. They were replaced and I load tested each battery in the string with an 11.5A load over several hours and removed approximately 75% of the capacity of each battery and extrapolated the remaining capacity. The one battery that was bad had the test terminated first since its cell voltage reached the termination point (10.88V) much sooner then the others about 3hours 20 minutes compared to about 5 to 6 hours for the others. They were all scheduled to be recycled so I "saved" the two best and recycled the others. The two going to the recycler were the ones tested.


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## dave.verry (Apr 5, 2006)

Also, open cell voltage is a crude approximation of the charge condition of the battery, and tells you nothing at all about the overall capacity. The only sure way of testing capacity is with a load test, which you do each time you have a load on the batteries and are off the charger. More reason you need a good battery monitor that will calculate amp-hours out vs. amp-hours in.


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## hellosailor (Apr 11, 2006)

Good points, but don't you have a wedding to get to?<G>

I suppose the "numbers be damned let's see what happens in the real world" way to double-check this would be to charge both cells, let them stand down, let them stand solo for a week, then load test them to see how many AH could be pulled out of each, singly. Maybe "practical amp hours" based on voltage drop or keeping a headlight lit.

Then recharge both, let both stand down again, leave them connected in parallel for a week, the repeat the load test, in solo, to see where the amps and volts had really gone. In theory...both tests should come out the same, if they're not consuming each other's power.

Maybe in the ten days till you get back, I can find out more. (You know us Missourians, "SHOW ME".<G>)


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## btrayfors (Aug 25, 2006)

Dave and Hello Sailor...

I think the proper test would be Watt-Hours or Kilowatt-Hours, not Amp-Hours.

These are a measure of total ENERGY....amp-hours is not!

It's quite possible to kill a "225 AH battery" by pulling out only 110 AH with, e.g., a 100-amp load. Equally so, it's possible to pull 400 AH from the same battery with a load of only 1 amp.

Bill


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## hellosailor (Apr 11, 2006)

Bill-
You are of course correct, we're looking at watt-hours but if we ASSume a 12v load being used and standardize the useful voltage...then amps is "good enough for gummint work". Give me a 55-watt 12-volt headlight as a test load, and I can measure in amps or hours or watts, and still get a useful number for comparisons. (The only question being, at what point you call the light too dim.<G>)


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## btrayfors (Aug 25, 2006)

Understood. And, you're right, the amperage doesn't vary a lot as voltage declines through the usuable range.

Here are some "real world" measurements from my ongoing pulsator tests. These were measured on a 2-battery T-105 bank with a load consisting of three 12V bulbs totalling 130 watts (rated).

VOLTS AMPS AH
13.11	10.36	0.00
12.48	9.92	5.10
12.40	9.88	27.20
12.28	9.83	49.30
11.92	9.67	102.80
11.52	9.50	148.40
11.14	9.34	174.20
11.04	9.28	178.80
10.88	9.20	183.40
8.69	8.20	190.20

As you can see, the amps only varied between 9.2 and 9.9 with voltages from 10.9 to 12.5.

Bill


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## btrayfors (Aug 25, 2006)

Oops...forgot the KWH column. Here's the whole thing:

VOLTS-AMPS -- AH -- KWH
13.11 -	10.36 -	00.00 (began load)
12.48 -	9.92 -	005.10 - 0.06
12.40 -	9.88 -	027.20 - 0.33
12.28 -	9.83 -	049.30 - 0.60
11.92 -	9.67 -	102.80 - 1.23
11.52 -	9.50 -	148.40 - 1.76
11.14 -	9.34 -	174.20 - 2.04
11.04 -	9.28 -	178.80 - 2.09
10.88 -	9.20 -	183.40 - 2.14
08.69 -	8.20 -	190.20 - 2.21

Bill


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## camaraderie (May 22, 2002)

Dave/Bill...thanks for the excellent work and discussion. Most helpful!


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## btrayfors (Aug 25, 2006)

Hey, it's fun. Hope it's useful, too, to dispell some wayward notions )

Here's a last note re: paralleling batteries...

Question:

Given the same house load (frig, lights, nav gear, radios, etc.)...

And given an installation consisting of two banks of two T-105 batteries rated at 225AH for a 20-hour load, with the banks wired through an A-Both-B-Off switch ...

Which situation will provide more total amp hours before the batteries are depleted (drawn down to 10.5V or whatever cutoff point you like)...

1. running them as two separate banks, one after the other; or 
2. running them as one large combined bank?

If you've followed this thread closely, by now you should know that the answer is #2, because you'll be putting a relatively smaller load (actually, half the load) on each of the batteries in the combined bank, and we know that the smaller the load the more AH can be drawn from a battery.

Yet another reason to combine 'em )

Bill


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## hellosailor (Apr 11, 2006)

Good point Bill. One still has to wonder:

What is the percent incidence of failure in batteries, from a shorted cell?

Because one shorted cell in one of two separate batteries, just kills one battery. One shorted cell in a paralleled set, will cause that 275A "short" between them that I mentioned come time ago. And leave you with no spare battery, unless you blew a primary fuse. Or, started a battery fire.

I get the feeling that's the kind of embarrassing question ("Hi, how many of your batteries short out and fail?") that no battery maker will disclose to the public.


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## btrayfors (Aug 25, 2006)

Cell failure through shorting, in my experience, is extremely rare on a reasonably well maintained battery setup. I have personally NEVER seen one. 

I have seen "weak" cells, and "bad" cells, but no real shorted cell that I can remember. It is, of course, possible (but so is it theoretically possible to vote in an honest and competent government....we just never seem to do it )

All house batteries should be fused at the maximum load expected. I believe my fuses are 125A...can't remember exactly..they're sized to be able to take a full charge from the 110A alternator or the 120A battery charger. So, in the VERY UNLIKELY EVENT of a 275A short, the fuse(s) would blow instantly.

I admit I haven't thought in any detailed way about the actual behavior of a shorted cell. Maybe it would just act like a short circuit between the two adjacent cells -- same as a strap -- and therefore would just not contribute its 2.14 volts to the whole???

Bill


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## hellosailor (Apr 11, 2006)

Bill, what's so hard about voting in an honest government? I usually vote five or six times in any election with no problem. Heck, I even know dead people who find it no problem to vote.<G>

" Maybe it would just act like a short circuit between the two adjacent cells -- same as a strap -- and therefore would just not contribute " Yeah, but then you've got a 10 volt battery hooked up to a 12 volt battery, and that just CAN'T be good.<G> Like the guy trying to waterski on the back of a Roman Trireme, he's not just "not contributing".<G>


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## dave.verry (Apr 5, 2006)

*I'm Back&#8230;*

This and some other questions have caused me to dive into my collection of papers on battery technology. I have found out some interesting items;

1.	AGM and Gel (sealed) batteries fail open most of the time (>80%). If you have a series only string, look out!
2.	Wet cells fail shorted most of the time. This is due primarily to plate growth and corrosion.
3.	Battery system reliability increases with paralleled batteries, dependent on a whole bunch of variables, but typically 10% improvement with each paralleled string. (88% for 1 to 98% for two to 99.5% for 3, etc.) This is true for wet and VRLA cells.
4.	Some paralleled systems have been used up to 16 different strings all on the same bus.
5.	Different capacity batteries that have the same chemistry can be paralleled with no degradation in performance or operational life.

And last, but not least, the current of between the two batteries under test (I left it going while I was away) after 2 weeks 1 day is 0.0000005 amps (0.5 micro-amps).

Citations for these conclusions are available, just ask.


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## btrayfors (Aug 25, 2006)

Good information, Dave.

Nice to have you back.

Bill


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## sailingdog (Mar 19, 2006)

Thanks Dave...


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## neilchristophers (Sep 14, 2006)

All good reading and fair comment from btrayfors with regard to switching banks at 3am so we must be better off with one bank ! as for parallel or series if this is a option series must be better as two cells in parallel will have different voltages and the higher one will allways be doing more work so will fail earlier, then leaving the job up to the other one on its own which will now fail before its time . Not what we want. Neil


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## dave.verry (Apr 5, 2006)

*Paralleling does not cause premature failure!*

Neilchristophers,

Not true. When the cells are connected together with cable they will be at the same voltage in all conditions. As they are charged the charger will impose the voltage and the batteries will accept the charge dependent on their capacity and charge state. If they are exactly matched (an impossibility, I know, but just for the sake of argument) they will accept the same current, if the charger can deliver 30 amps, both will have 15 amps going in. As the charger reaches its upper voltage limit (boost or float charge) the current will taper off in both batteries until the current counterbalances the self discharge of the batteries (full charge).

The same happens if the batteries are not matched. The difference occurs in the amount of current each battery accepts curing the charge cycle. If one battery has twice the capacity of the other (through wear-out, or even if two batteries of different capacities are used), and using our 30 amp charger, the larger batter will accept twice the current of the smaller one (20 amps for the larger, 10 amps for the smaller). The same taper will occur as the final voltage is reached and both batteries will reach full charge at about the same time.

In the discharge mode, a similar thing happens. As you put a load on a battery, the internal resistance causes the output of the battery to drop immediately. This drop is small so if you try and measure it you will need a voltmeter to several decimal places, preferably measuring 1/100 of a volt @ 12volt range (this would be called a 5 and ½ digit meter). The important point is that this voltage drop allows the current of parrelled batteries to be shared _in spite of their relative capacities!_ Taking out two examples the exactly matched batteries will exactly split the load current equally, and our mismatched batteries will have the current from the larger one at twice that of the smaller one.

This equates to the load currents of parrelled batteries as having the same relative current based on their capacity. Both batteries will discharge at the same rate (but with different currents) and have the same state of charge at the same time, and reach the end charge state (weather it's 50%, 25%, 100%, or what ever) at the same time!

So, parrelled batteries charge at the same time and discharge to the same relative state of charge at the same time. Each batter will do the same relative work based on its own capacity. If one fails faster then the others, it would have failed in the same time if exposed to the same duty even if used independently. Paralleling batteries does _NOT _cause them to fail prematurely.

BTW, this conclusion is based on many, many scientific papers I have been using in the course of my work, and that I have reviewed while and after I have been posting on this thread. I will make them available to any who ask.


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## ralphhurley (Nov 12, 2006)

Dave,
I'm trying to use four 6 volts to replace my house bank. When you say " parallel" do you mean that the batteries are connected pos to next battery's neg pole? If so, does that continue on to battery 3 and 4?This is as opposed to series which would connect all four pos poles and all four neg poles seperately?


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## btrayfors (Aug 25, 2006)

Ralph,

I'm assuming that you have a 12V system, and that you want to use four 6-volt batteries for the house bank.

Essentially, you make two 12-volt batteries by connecting two batteries in series (positive to negative). THEN, you connect the two 12-volt batteries in parallel (positive to positive).

Example:


1. Connect the first battery's positive pole to the second battery's negative pole. This will give you 12 volts, measured between the two unconnected poles.

2. Do exactly the same thing with the third and fourth batteries. Now, you have two 12-volt battery banks, each consisting of two 6-volt batteries in series.

3. Now, connect these two banks in PARALLEL, by connecting positive to positive, negative to negative. You now have one large 12-volt bank consisting of four 6-volt batteries in series/parallel.

This combined bank will have twice the capacity in AH of each individual bank. If you're using golf cart batteries with a nominal 20-hour rate of 220AH, then you will have a 440AH house battery bank.

If you have any doubts or uncertainties about this, ask a knowledgeable friend.

Bill


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## dave.verry (Apr 5, 2006)

Bill is correct, in essence you are making a 12V battery.

There are two ways, actually. The first is as Bill described it, putting two 6V batteries in series (positive to negative, creating one "string" of 12V, and connecting the two positives and the two negatives of the strings together [the posts not connected to the other battery in the "string"]) the second is to parallel two 6V batteries together, twice (positive to positive, negative to negative) so you have two of these assemblies, and then connect those assemblies positive to negative to form the 12V "string".

Use whichever way best fits your boat and how the batteries and wiring fit.


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## sailingdog (Mar 19, 2006)

The only problem I see with this Dave is that the second configuration is a bit more prone to failure than the first. 

From what I see... if any battery in the first configuration dies, you have a 12V and a 6V battery, which may or may not be connected, depending on the failure. On the second one, if a battery fails, you will definitely have a connection between the 6V and the 12V battery. 

However, I am a bit rusty on my electrical circuit theory, since its been over 20 years since I've used it regularly.


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## dave.verry (Apr 5, 2006)

Dog,

Thank you for bring up this point, I was remiss in not explaining the difference.

Correct, there are two additional single points of failure, the connections between the 6V blocks (discounting the wire and crimps). The first configuration has redundancy, the second does not, so the first is preferable. Increased reliability can be accomplished by feeding both strings to the same terminal point, instead of one string to the other. 

At some point the amount of redundancy in your system depends on how much you want to take it. At the extreme you can have two totally independent systems duplicating everything including loads (electronics, lights, refer, etc.). At some point you have to decide how much redundancy to have.

Don’t get me wrong, I have spent a lot of time on this thread arguing for paralleling batteries as a means to improve reliability and capacity, and would advocate doing it the first way, but in the end it comes down to personal preference.


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## CharlieCobra (May 23, 2006)

Helluva thread.


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## bhhewett (Nov 28, 2000)

*This discussion has produced a conclusion...*

Gongrats all, on a helluva thread. Much dialog (knowledgable to boot) on a much needed subject. After reading and digesting it all I've come to my own conclusion and will implement it forthwith in my old 35'. I agree with the premise that one large bank (four or six Trojan 105's) is a better setup, with a completely separate engine bank. One master on/off for each bank. In cases (hopefully not) that either goes flat I'll simply carry a heavy duty set of cables for emergency jumpering.
I believe it will be easier to monitor one large house bank (using a Link2000 or similar) than wiring 2 separate house banks, associated switches, wiring, yadayadayada, and monitoring both, not to mention remembering to switch back and forth...I've got enough other things to remember, like where I stashed my spare Racors...
Cheers, and thanks!


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## DeepFrz (May 9, 2006)

One of the most informative threads I have read on paralleling batteries. Thanks everyone.


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