# Cost Per Amp Hour



## Snboard976 (Aug 10, 2008)

I need to upgrade my house battery bank. I am looking for the most capacity for my money so I am pretty well set on Wet-cell batteries. Can I comparison shop by looking at amp hours per dollar, or is there another spec I should also pay attention to?

Right now I am looking at getting 2 or 3 sportsman group 31 12volt batteries from defender. Each is rated at 140 ah for $121 (86 cents per amp hour.) Is there a better solution for me?

My other dilemma is that I have an alternator on my 1976 Westerbeak that is rated at 55 amps. West Marine advises that the alternator be no less than 25% of the total amp hour capacity. With two group 31s my alternator will be about 20 percent of the amp hour capacity. Is it worth it to add a third battery. How long would I have to run my engine to charge them?

I will be cruising throughout the Caribbean for a year spending almost all of the time on anchor. The boat is a Pearson 365 and will have all of the typical power demands + laptop + radar + occasional use of a small TV. I have about 1000 dollars to increase our power supply do you have any other ideas?


----------



## scottyt (Jul 19, 2008)

a lot of people like the walmart deep cycles, for 61 bucks for 115 amp hours they are cheaper, infact i plan on 3 new ones for my boat in the spring.

as for charging time, the old saying is you will get 75% charged in 25 % of the time. say with a 230 amp hour battery at 50% discharge about 115 amps ( ie 2 walmart batteries, at 50 % discharge for max battery life), which a 55 amp alt will put in 55 amps or so the first hour, so a 50 % charge of 115 amps. the second hour you will put about another 20 to 30 amps in, so you will be up to about 70 % charge or so, then the extra 30 % might take 6 hours. the reason is as the battery gets closer to 100% it accepts less charge amperage. 

if you go to 3 batteries at about 350 amps you will want a bigger alt or lots of wind and solar charge. face it at 350 amps ( 175 useable amps ) a 90 amp alt after 2 hours will put in about about a 140 or so amps. thats why 10 amps of solar ( about 150 watts of panel ) is so needed in 8 hours of day light you will get about half of of your charging done with no fuel use


----------



## scottyt (Jul 19, 2008)

second post i would recommend at least 350 amps of battery, so your useable amount is 175 amps ( 50 % of capacity ). that should get you two days of normal use before any charging , then 150 watts of solar, plus a honda genny for weekly topping of the batteries with out using the main engine. this should have you only needing to use fuel once a week if you are frugal, because of the solar


----------



## sailingdog (Mar 19, 2006)

Just curious as how you determined he could get two days of use out of 175 amp-hours of bank capacity, since he hasn't said a thing about what his electrical budget is...

Also, with a budget of $1000, getting 150 watts of solar, four batteries and a Honda generator is not going to be possible, unless he steals some of it. 



scottyt said:


> second post i would recommend at least 350 amps of battery, so your useable amount is 175 amps ( 50 % of capacity ). that should get you two days of normal use before any charging , then 150 watts of solar, plus a honda genny for weekly topping of the batteries with out using the main engine. this should have you only needing to use fuel once a week if you are frugal, because of the solar


Snboard-

You really need to compare amp-hours/$, but make sure you're comparing apples to apples. Some batteries give the amp-hour rating for a 5-hour use rate, others for 20-hour, others for 25-hour... so make sure the amp-hour ratings are for the same time period.

Personally, I like the Trojan golf cart batteries, as I believe they're probably more durable than the wal-mart batteries. Four of them would give you 450 amp-hours at 12VDC, with two-pairs hooked in series, then connected in parallel. With batteries, you often do get what you pay for.


----------



## xort (Aug 4, 2006)

Trojans are available everywhere. They have many different configurations to fit the space available and are well respected for durability

how much will it cost to ship those batteries from defender?

you will have longer charge times with a smaller alternator. You might be able to swap out for a bigger 'small frame' altermator, up to about 100 amps.
What about the regulator? Do you have an external regulator? it just keeps adding up!!


----------



## Rockter (Sep 11, 2006)

The regulator you use will be important. A fixed voltage regulator is likely to load that alternator heavily as the fixed charging voltage does not really take into account the battery charge state. 

The so-called 3-step regulators are a better product, but they can be expensive. 

Also, be wary exactly what they mean by a 55 A alternator. Is that continuous rated load? If so, at what cooling air temperature? 

16 years ago I bought a small frame 100 A alternator from Ample Power and a 3 step regulator. The alternator, though expensive (then perhaps $300, and the 3-step regulator perhaps $100?) was rated 100A CONTINUOUS at a max cooling air temperature of 91 degC ( 196 degF !!!). I was sceptical at the time, but it is still running. I have seen it hold 60-80A for perhaps 15 minutes and manage it. The alternator output steadily drops as the charge of the batteries recovers, so I can't state a single figure. As a rule, I let this alternator breathe when it is working hard.... I leave the engine cover off the front of the motor and don the ear defenders.

I had experience of another manufacturer's alternator, seemingly rated to 100 A, and it blew in less than an hour, pulling perhaps 1/3 of that. 

I have no interest in Ample Power other than saying that the alternator worked for me, and it still works.
.


----------



## scottyt (Jul 19, 2008)

sailingdog said:


> Just curious as how you determined he could get two days of use out of 175 amp-hours of bank capacity, since he hasn't said a thing about what his electrical budget is...
> 
> Also, with a budget of $1000, getting 150 watts of solar, four batteries and a Honda generator is not going to be possible, unless he steals some of it.


i said normal use, which in his post he said lap top, radar, small tv occasionally. i also expect things like lights radio etc. he did not say fridge or water maker, so his use will be slightly above minimal but not extreme IMHO. if he was using just lights and radio his use would be around 50 amp hours a day from my experience, the tv would not use much becuase as he said it would be occasional. the radar and laptop are the only things above minimal use i saw, and the laptop if a newer one would only draw about 10 amps a day ( based on 2 amps 5 hours a day ).

basicly i said what i said ignoring budget to show what he may need. also the recharge every 2 days thing is a slightly educated guess, based on what he said and what i know.

SD what would you calculate based on lights ( on anchor not running at night ) small tv occasionally, radio and a newer laptop, but no fridge and no watermaker?

edit if people read around lots of people use the walmart batteries, and at 50 % of the cost if they last 3 years it is a break even or slightly ahead, but a savings of about 200 bucks now when he needs the cash


----------



## timebandit (Sep 18, 2002)

how does one start a new thread?


----------



## funsailthekeys (May 15, 2008)

SD was very correct but you must consider the cycles the battery can last. A good deep cycle battery can go fully dead to fully charged about 300 times. Cheaper batteries may only last 100 or less. This is where the cost of amp hours comes in. The batteries may cost less but not last as long.


----------



## erps (Aug 2, 2006)

We went through the math on battery amps and cycles and the best value were 2 volt industrial batteries. The downside, a hugh initial investment and they're big. Our 600 amp bank was over $1000 back in 1999, but they're ten years old and still going. I don't expect to get another ten years out of them though.


----------



## Snboard976 (Aug 10, 2008)

Thanks for the advice. Wallmart batteries an idea. I couldn't find them in a search of the website. Does anyone have a link for them?

It looks like I will get 2-3 batteries and spend the rest on a wind generator.


----------



## scottyt (Jul 19, 2008)

snboard i looked last night they dont seem to have em on line. but they are a group 27 115 amp hour price about 62 bucks. they have a larger one but the price goes way up for not much more. 
another idea is costco/ sams have golf cart batteries they are 6 volt 230 or so amp hour, for about the same price. the problem is they have to be wired in series so if one battery dies you stuck with 6 volts. why not give us a break down of what you have including what type of lights. 

as for dead to full charged life cycle, we all know the idea is never go below 50 % discharge. if he has enough charging ie solar or wind ( what he posted ) he should not get under 25 % discharge ( 75 % left ) anyway except rarely so it becomes a mute point.


----------



## scottyt (Jul 19, 2008)

erps said:


> Our 600 amp bank .... .


erps what does you load look like, water maker, fridge etc... and whats your charging? ie solar wind genny and how often ?

i am just curious to see what you are running and how often you need extrenal charging


----------



## erps (Aug 2, 2006)

Our load is around 50 amps/day which is primarily the fridge in a well insulated box. We have two alternators on the motor, one for the starting battery and one for the house bank. We're putting on a couple of solar panels this year or next year, about 200 to 240 watts worth and we carry a Honda 2kw generator. In the tropics I figure our dailey amp load will go up to around 80 amps/day. We do not have a water maker and I haven't made up my mind on that yet, but we carry 160 gallons and others I've talked to with that kind of water capacity get by without one. A downside with six 2 volt cells is that if one cell goes bad, you can lose the whole bank. The upside is if one cell goes bad, you can replace just that battery, at least that's what I've been told.

Oh, and in their cycle ratings, they included cycles at 50% and 80% and the 80% rating was still better than some of the cheaper golf cart batteries IIRC. Good batteries, but only if you're going to keep the boat for their life span, otherwise you're just giving money to the next owner.


----------



## sailingdog (Mar 19, 2006)

scottyt said:


> SD what would you calculate based on lights ( on anchor not running at night ) small tv occasionally, radio and a newer laptop, but no fridge and no watermaker?


I wouldn't even hazard a guess, as the use of LED lights, like the OGM LED anchor light, Sensibulb cabin lights, LCD TVs and sub-notebook laptops will all have much lower draw than incandescent anchor light, halogen cabin lights, a CRT-based TV, and a 17" full-multimedia gaming notebook... Too many variables.


----------



## timebandit (Sep 18, 2002)

Has anyone considered a 50 amp A/C altenator conected to the engine by say a clutch pulley like the one used on and automotive air conditoner?

The drive system would be geared to let the engine run just above an idle since the only load would be the altenator and the engine cooling system.

When motoring at a higher rpm the regular charging system would be used and the A/C altenator desengaged.

It seems that the hardest half of an on board genset is allready there (water cooled engine and accessories) and all that is needed is a generator.

50 amps at 12v = 600 watts
50 amps at 110v=5500 watts

You could just induce the power output of the A/C alternator into the shore power system for ease of wiring.

All of the dock ammenties would be avaiable and great battery charging too while the engine would just be running the same amount time it would take to keep the batteries topped off.

A sailboat engine running just above and idle warming up is pretty quiet compaired to one running fast enuff to put out 50 amps of D/C.

Wadda think??


----------



## scottyt (Jul 19, 2008)

it could work, but one again the cost vrs rebuild time i would think a honda 2 k would be cheaper. the engines in our boats dont like to idle, its bad for em. 

the best idea i have seen was on an old gulf star i did some work on. it was a pull started single cylinder deisel with a dedicated 150 amp alternator tied to an inverter. it could feed the 110 volt wiring and the battery for rapid charging. but space is an issue, the engine bay on that boat was big enough for 3 people.


----------



## timebandit (Sep 18, 2002)

it could work, but one again the cost vrs rebuild time i would think a honda 2 k would be cheaper. the engines in our boats dont like to idle, its bad for em. 


So what do curisers do when they use the D/C Altenator to charge the batteries--Run the enging at cruise rpm?

You could always gear the drive for tha engine rpm.


----------



## erps (Aug 2, 2006)

We have a 110 amp alt on a single belt and we wear out belts. That's equivalent to 110 x 14 volts = 1540 watts. Wouldn't trying to get 5500 watts be pushing a belt driven system?


----------



## Maine Sail (Jan 6, 2003)

timebandit said:


> Has anyone considered a 50 amp A/C altenator conected to the engine by say a clutch pulley like the one used on and automotive air conditoner?
> 
> The drive system would be geared to let the engine run just above an idle since the only load would be the altenator and the engine cooling system.
> 
> ...


While the small case and large case marine alts are indeed AC internally there is no commercially available AC regulator for use with these small alts.

In order to use an AC generator on a boat it would be rather large.

My 5500 watt Generac generator, for home use, is a 13 hp motor and the generator portion is about four to five times as large physically than is an alternator.

Robbing 11 horsepower to produce 5500 watts of power on a small marine diesel at cruise rpm is going to be more than 50% of its total capacity. A Westerbeke 5.5 kw or 5500 wat generator consumes 11 hp at 1800 RPM to produce 5500 watts...

For instance on a two cylinder Westerbeke 20B Two it produces 18hp at 3600 rpm but only 10hp at 1800 RPM. This is not even enough to supply and power a 5500 watt or 5.5kw generator let alone motor out of an anchorage under power.

That same engine at rated cruise RPM is about 16hp so a 5500 watt load would consume nearly 70% of the total available power to the prop leaving you only 5hp left over motor with at full cruise RPM..

There is a reason we use the alternator systems we use on small diesels..

Even if you could find one you would most likely need to direct drive it as is done with gen sets.


----------



## timebandit (Sep 18, 2002)

Without seeing your set up I can only give you some design parameters that might solve your problem.

To get good power transfer from a belt system you should have at least 50% belt wrap (belt to pulley) on each pulley the more the better.

I'm guessing that because of space limitations you have less wrap on the lager crank pulley and even less wrap on the smaller alt. pulley, this is of course assumeing that you are just driving the alt. alone (crank to alt.).

You can---

A) Be sure you have the correct belt. (industrial or auto)
Belts transfer power on the sides so if your pulleys are shinny on the bottom of the grove the belt is either wore out or the wrong one or the pulley is wore out. Belts should not run below the top of the pulley.

B) Widen the center distance between pulleys(prolly can't due to space constrictions).

C) Run two belts. In the 60's the police intercepter engines all used 2 belts to run a 60 amp alt. I bet your alt. came with this option.

D) Increase the diameter of the alt. pulley. You will have to run the motor faster but as the amps drop you can throttle down.

E) Add a back sided idle pulley to increase the wrap on alt. (Goes on the slack side)

Any of these sugestions will not require extra tension on the belts so the alt. bearings will not suffer and the belt should last longer.

For a 5500 watt system I would prolly use a polly v system or cog (timming ) belt. And I guessing that you would not use all 5500 watts much.



erps said:


> We have a 110 amp alt on a single belt and we wear out belts. That's equivalent to 110 x 14 volts = 1540 watts. Wouldn't trying to get 5500 watts be pushing a belt driven system?


----------



## motion300 (Feb 12, 2009)

I have a 100amp alt with a single 1/2'' belt No belt issues. My motor turns 1700rpm at cruise In my research I found a 1/2'' single belt will turn 100 amps above that double belts are required


----------



## hellosailor (Apr 11, 2006)

"C) Run two belts."
Gotta disagree, Rick. With dual belts there was ALWAYS a problem in the taxi/police cars, because even with the best of care the two belts don't share the load equally. Then one snaps, both need to be replaced and the cycle starts again.
What has proven to be way more reliable is using just one ribbed belt. That's why v-belts just aren't used in cars any more, they're getting hard to find in the parts stores, too. Of course, that may mean remachining the flywheel on a boat engine--which is usually old enough to still be set up for a v-belt. But if you've got to change the flywheel anyway...might as well go to the better belt.


----------



## erps (Aug 2, 2006)

Doh! I think there's been a miss-communication. 

In a previous post, it looked like someone was trying to get 5500 watts out of an alternator tied in with their A/C system. I didn't think that was possible, but instead of pointing it out directly I was pointing out that in my particular case, I was getting 1540 watts out of a single belt setup and that was taxing a single V-belt system, basically confirming what motion300 was saying (100 amp max for a V-belt)

On our work boat, we have a 100 amp alternator run by a ribbed belt. Even that squeals at times when we use the inverter to make coffee.


----------



## Bene505 (Jul 31, 2008)

Snboard976 said:


> I need to upgrade my house battery bank. I am looking for the most capacity for my money so I am pretty well set on Wet-cell batteries. Can I comparison shop by looking at amp hours per dollar, or is there another spec I should also pay attention to?
> 
> Right now I am looking at getting 2 or 3 sportsman group 31 12volt batteries from defender. Each is rated at 140 ah for $121 (86 cents per amp hour.) Is there a better solution for me?
> 
> ...


This feels a little like that guy in the movie Apollo 13, that had to keep the amps below 20 amps in the simulator. Only now it's dollars below $1,000. Here goes... Oh, and this is all my opinion, of course. IMHO...

First, use whatever you have. Keep the 55 amp alternator. Keep the batteries you currently have, just put them in series with the new ones. (Unless they are damaged or shorted out.) (And unless CD says you can't do that.)

Next, look at what you will be using per day typically. Buy enough batteries to bring yourself to 2 times that usage. That's the bare minimum. If there's left over money after the next steps, buy more batteries.

Let's say that's 2 batteries on top of what you have (could be 2 6V batteries). That's $120, you have $880 left.

Get a used Air-X wind generator off eBay or craigslist, the kind with the built-in regulator. They are a bit noisy in high winds, which may work to your advantage, since you are buying it used from someone who didn't like that. Get one of these that are posted on craigslist right now:
Wind Generator
Air-X Marine Wind Turbine Generator for your boat
Air-X Marine 400W Wind Generator with Mounting Poles

Talk them down to $600. Pay for shipping (guessing $40).

You have $240 left to build a mast for the Air-X. (Or for a little more, get the one on ebay right now. It's $295 + $55 shipping.)

If you do build your own mast, you may have some money left over for that 3rd battery.

Finally, no matter what you do, let us know and send pictures! Have a great trip.


----------



## Snboard976 (Aug 10, 2008)

So here is the current plan pretty close to Bene's suggestion. Please let me know if you see any potential problems or something that could be done better. 

I found the Everstart Marine Max at Wal-mart. They are rated at 125ah each. I plan on getting 3 of them this weekend. West Marine online adviser says this is too many ah's because I only have a 55 amp alternator. What do you think?

For additional charging power I am looking at a low cost wind generator. The three in my price range seem to be an air-x, air breeze and KISS. I am not a big fan of an unregulated generator free wheeling in high winds, so I think that will rule out the KISS. The air x has a higher output potential in high winds, but a higher start up speed than the air breeze. I thought the Air-x would be better in the Caribbean. What is the avg. wind speed which tips the balance in favor of an Air-x over an Air Breeze? Any other low cost wind generators I should look into? I am not concerned about noise from the generator. I promise I won’t anchor too close to anyone. Once the boat is launched Memorial day I will work on securing the wind generator to the mizzen mast.


----------



## hellosailor (Apr 11, 2006)

3* 125Ah = 375Ah fully discharged, but only 187.5 Ah if cycled to the typical 50% point. A 187.5Ah wet cell battery bank would take an optimum charge rate of about 46 Amps, which would work out just fine. Except, your alternator is probably internally regulated and designed to cut back very rapidly to a fraction of that, or overheat from the sustained higher power output.

So it might work, particularly if the alternator was properly cooled and externally regulated so it could put 45-50A into the batteries during charges. 

You'd be better off replacing it with a larger alternator (i.e. 75A-90A) and an external regulator, and putting your old 55A aside as a spare.

Of course, if you don't put the 3 batteries into one bank, the numbers become more conservative and you can get away with that alternator, but then you have to deal with manually switching more battery banks more frequently.


----------



## camaraderie (May 22, 2002)

375 amp hours is just about 15% capacity which is just FINE for charging. 20% would be perfect but anything more than that is a waste. A 375ah battery bank will only accept up to about 75 amps in bulk and that will taper off. As HS says...75-90 would be ideal but 55 will be fine...no sense spending more on a tight budget. I suspect that the Walmart batteries are actually LESS than 125ah's on a 20 hour basis in the real world....so your alternator will be even better matched. 

The KISS is the best of the three. 10 knot performance is ALL that really matters in a wind vane. THAT is reality in most anchorages most of the time IF that. Fun to load everything up on those 20 knot days...but those are not frequent in Caribe anchorages.


----------



## Bene505 (Jul 31, 2008)

If you are mounting on your mizzen mast, are you going mount it directly (so it only works while you're at anchor, pointed into the wind)? Or are you mounting a stand-off so it can swivel 360 degrees?

It it's the former... IIRC, there are some very cheap wind generators that mount with no swivel. I think they use a car alternator. I haven't used any of them or seen what happens to them over time in a marine environment.

Please remember to send pictures. Do you have any of your boat and your current battery setup?


----------



## Bene505 (Jul 31, 2008)

I hesitate to post this becasue it's probably out of your price range.

Here's a nice description of the Fourwinds II by some people who cruised from Texas to near Panama. I found this by accident while looking for a picture of one. I'm installing mine once other boat projects get done. It's probably too expensive and you don't really need quiet.

Fourwinds II Wind Generator

And their boat:









.

Remember craigslist and ebay are your friends. (Hint: check out crazedlist.com)
Good luck with whatever you choose and looking forward to hearing about your adventures.


----------



## Snboard976 (Aug 10, 2008)

Thought I would follow up with my end result. Thanks to everyone who offered advice.

3 x $80 Evermax Marine Batteries (115 ah) from Walmart. At 70 cents per ah they were the best deal I could find.
1 x $65 Marine Starting Battery (lots of cold cranking amps) from Auto Zone 
1 x $750 Air Breeze Wind generator (got WM to price match another vendor.)

So in the end like most things on the boat I have gone over budget and I forgot to include the mounting brackets for the wind gen. or the additional cables. Oops. I should have plenty of Ah after I get the Wind gen up next weekend though.


----------



## Maine Sail (Jan 6, 2003)

Snboard976 said:


> Thanks for the advice. Wallmart batteries an idea. I couldn't find them in a search of the website. Does anyone have a link for them?
> 
> It looks like I will get 2-3 batteries and spend the rest on a wind generator.


The group 29/31 Wal*Mart deep cycles are rated at 125 ah's and now run about $70.00 to $80.00 depending upon the store. Not all Wal*Marts sell at the same prices..


----------



## Waltthesalt (Sep 22, 2009)

On charging batteries, you may want to get a smart amp-hour meter to manage discharge and charging. One problem is that to finish the battery charge you'll be charging at a low rate for a long time. Not good for the diesel if unless your also running it at load. There a solar panel can effectively take over the charge. For a pretty thourugh discussion on this google the Bogart Engineering website and look at the PDF literature associated with thier Tri-Metric battery monitor.


----------

