# Batteries Won't Charge from Engine Alternator



## efiste (Apr 5, 2000)

Tried searching the forums here, various Internet sites, and researching through the Don Casey and Nigel Calder books... but I cannot figure out my issue.

I purchased a 1987 Catalina 34 (as a replacement for a 1983 Hunter 28.5) this past winter and delivered it from Whitehall, MI to Chicago the first weekend this month (May 2011). I made 3 stops along the way at Grand Haven, South Haven, and New Buffalo before heading across to Chicago. Everything went well, but on the 2nd leg from Grand Haven to South Haven (longer than the first leg) we noticed the voltmeter at the engine instruments had dropped below 12V even though the engine was running most of the day (6+ hours) due to head on winds. Next, we noticed the tachometer acting erratic and suspected a loose belt. The belt was tightened that evening, and we assumed the problem was solved. The next day, though, we had continued battery issues. After charging on shore power the 2 batteries (both West Marine Group 24 12V Starting Batteries 1000 MCA) seem to have around 13.5V. When heating the glow plugs on the Universal Diesel M-25XP on both batteries (or either using a battery isolation switch), the volts drop down below 11V until the engine is started. When the engine is started, the volts go back to the charged "battery" level of around 13.5V. Throughout the day (engine running or not), the volts slowly drop while using various medium load electronics on the boat. My understanding is that with the engine running, the alternator (Motorola 55AMP internally regulated) should be producing roughly a volt more than the batteries to charge the batteries under power with minimal load. I, however, get no such "jump" in volt readings from when I turn the key to on and then start the engine and the volts slowly drop. I ran some tests with a multimeter, and received some results which seemed to indicate that the alternator may not be operating correctly (diodes shot or faulty regulator). I removed the alternator and took it to a good auto electric shop in the area. They tested it and said there were no problems with it mechanically. I am at a loss.

As far as I know from the previous owner and yard mechanics, there were no issues last year. The previous owner, though, was at a slip with shore power which seems to charge the batteries fine. I am on a mooring in Chicago, though, without power access.

Is the problem likely with the alternator (already checked), batteries (will charge with shore power; no different starting and house load type), wiring (what I can trace seems good), etc.? Thoughts, questions, advice? Any assistance to avoid calling out a mechanic to service the boat in the water would be appreciated.

Eric


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## jaybeegee (Dec 27, 2010)

Hi! Have you also checked that the batteries are ok? A car battery shop will tell you instantly using some testing device they have.


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## Traveling Light (Dec 2, 2002)

If the batteries hold a charge from an AC charger and the alternator checks outs then it may be a very simple problem. For example, I once "messed up" because I did not run a wire from one battery to the other to complete the circuit. It may be a bad electrical connection, or the batteries are wired up incorrectly. Are your terminals clean? Are the cables OK?


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## MarkSF (Feb 21, 2011)

The alternator won't produce much more than 13.5V so if the batteries are at 13.5V when they have just been charged by the shore charger, you can't see the effect of the alternator. What you need to do is test the alternator with slightly discharged batteries. 

I suggest disconnecting the shore charger at least 24 hours before, then try it. Alternatively you could turn off the charger and turn on some items (lights are good) for 1/2 hour.

Either way the resting voltage of the batteries should have dropped to 12.5V or so, in which case you can now test the alternator. Start the engine. The voltage should go up to 13.7 or so and stay there while the engine is running. If not, the alternator, or the wiring to it, is faulty. If the alternator tests fine then you could have a wiring issue.

Please report back on what you find.


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## shogan50 (Jan 2, 2011)

A few thoughts. 1. Mine charges at 14.4-14.6 at anything above idle. 2. 11V at the battery after 10 seconds of glow plug use, sounds to me like batteries might be pretty tired. 3. I recently had flakey charging symptoms that turned out to be loose wing nuts on battery connections. 4. Is your exciter wire connected/corroded?


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## efiste (Apr 5, 2000)

@Traveling Light - I believe the batteries are connected correctly. This is a task I completed on my own, but used the context of how the cables were tied together and laying in the battery compartment along with the owner's manual to confirm. Everything has been working fine regarding how the DC electronics operate on either position 1, 2, or ALL of the battery isolation switch. However, I did have a situation the other day that I could not quite figure out. I removed 1 battery from the boat to recharge from home. I did not plan to connect the disconnected batteries wires (here is where my lack of electrical expertise will start to shine), but I could not get any DC power to the cabin lights (for example) with either battery bank selected. In this case, I believed the battery still connected was battery 1. I thought since one of the large #4 reds and the single large #4 black were connected to that battery, I should be good to go. I have other smaller wires in the battery compartment, but they are for the sump float and shore power battery charger (both of which were not in play for my unexpected results). I did not have a lot of time to complete my testing, so I had to connect all of the wires to the single battery to get power. Now, I realize that there are many factors in play here, but I would say that in general this boat has had pretty good care. Therefore, I would assume my error (or lack of knowledge) over the boat having something wrong. It has been a few years since my last boat. I have attached the general wiring diagram for the Catalina 34 as reference. I did indeed connected the black #4 tie between the 2 batteries.

Eric


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## efiste (Apr 5, 2000)

@MarkSF - After running the engine for many hours (6+ during 2 of the trip legs from MI to IL), the voltmeter at the engine instruments never "jumped" to ~13.5V (as expected when the batteries where not at top charge) and in fact slowly went down to <12V as we motored on. I think this is a symptom of the problem, but I am not sure of what since the alternator checked out.

Eric


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## efiste (Apr 5, 2000)

@shogan50 - The <11V is when actually prepping (warming) the engine prior to starting. After the engine has started, the voltmeter at the engine instruments returns to the battery charge level (minus a little each time I start the engine). In other words, it is really like the alternator is not producing power (which it does), the batteries are not able to be charged (which they are), or something in between is a problem.

BTW, the cabling all looks good, but I will need to give it another once over since the alternator came back with no issues. Also, the batteries are newer (both starters no deep cycle as mentioned) but were not maintained over the winter. I have one connected to a house charger now. The charger is NOT indicating any deep discharge.

Everyone's help is greatly appreciated.

Eric


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## Traveling Light (Dec 2, 2002)

If you are new to the Chicago Harbors I suggest you contact the Chicago area Catalina fleet. My boat (Pearson 28) is in Monroe Harbor and I'll bet that there are lots of people in the club and at Monroe that will coming to your assistance. 
Catalina sailing in Chicago I used to belong when I had a Catalina and they are very nice people.


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## kd3pc (Oct 19, 2006)

alternator has a fault or the regulator is toasted...


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## MarkSF (Feb 21, 2011)

efiste said:


> @MarkSF - After running the engine for many hours (6+ during 2 of the trip legs from MI to IL), the voltmeter at the engine instruments never "jumped" to ~13.5V (as expected when the batteries where not at top charge) and in fact slowly went down to <12V as we motored on. I think this is a symptom of the problem, but I am not sure of what since the alternator checked out.
> 
> Eric


It's a symptom of a faulty alternator. Whether it checked out or not, it's not charging when it's in your boat. The possibilities are :

Faulty wiring to the alternator
Faulty alternator
Alternator not turning
Faulty batteries
and, last but not least, faulty voltmeter. Try a DVM on the battery terminals.

I've not heard of a battery fault that agrees with your observations though.


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## NCountry (May 25, 2006)

If the battery is under load when you check the voltage it's fairly common for it to read less than fully charged. Especially if it's not been fully charged. Once the load is taken off then the battery should return to reading fully charged. See if you can get all the batteries "Load Tested". They could read fully charged but still not carry a load for any length of time.
Next, check all the connections including the connections on the back of the alternator. While checking these connections run your hand along all the wiring and squeeze it between your fingers. If you hit a "soft spot" you've found the problem. I've seen wiring be bad inside without the outer insulation showing any signs of any problems. The other problem that actually turns out to be fairly common is undersized wiring. On my current boat I about went crazy looking for a problem. Everything, batteries, both alternators, charger all tested out good. Problem turned out to be undersized wiring. The manufacturers only put a size of wiring on the boat to handle what they've installed with very little additional equipment. If the PO is like most normal sailors they immediately start adding "stuff". All this "stuff" requires electricity. This places additional load on the electrical supply system and can lead to bad wiring inside the insulation with no apparent signs of problems.
I'll be the first to admit that I must really be stupid on this electrical stuff but as soon as I replaced a lot of the old wiring with new heavier guage wiring EVERYTHING started working exactly like it was supposed to... It was almost miraculous!


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## hellosailor (Apr 11, 2006)

I'll go out on a limb and say that IF the alternator and internal regulator are good (and since they were tested, supposedly they are) and IF the batteries are good (and they take a charge so they should be "good enough" with a running engine)...that would all point to the alternator not being turned on when it is in the boat. Meaning the exciter wire iss't exciting the alternator, assuming a standard hookup.

Check for a plug with 2-3 wires in it coming off the alternator, not the primary charge output which should be separate. One of those should be the excitor wire, which is connected to the battery +12V when the key is turned on. If that wire has been disconnected somewhere along the way, that would keep the alternator "turned off". Some setups also have an alternator warning light in the instrument panel, and sometimes the light bulb in that warning light is actually a necessary part, providing a limited current to the alternator. If the bulb burns out--again, the alternator is turned off. Usually those bulbs all light up as a test when you turn the key without starting the engine.

It could just be one of those simple things in the alternator "control" wiring. Worth checking them out against a schematic for your alternator, before you call in a mechanic. (Who'll probably tell you to call an electrician, or pull the laternator to test it again.)


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## Stu Jackson (Jul 28, 2001)

Your OEM Motorola alternator doesn't have the external regulator plug that hellosailor mentioned.

The wiring at the back of your alternator is discussed here: Regulator to Alternator Wiring and a link in Reply #1 goes to the wiring diagrams. I sketched these when I replaced our alternator.

I do agree, however, that it is either the excite wire specifically or could be any one of many connections, including the ones at the back of your 1-2-B switch.

For more disucssions about wiring and electrical, please feel free to peruse our C34 message board, like here: Electrical Systems 101

Many of us use more than one message board. With your C34, you are welcomed to join us there anytime.


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## Omatako (Sep 14, 2003)

MarkSF said:


> It's a symptom of a faulty alternator. Whether it checked out or not, it's not charging when it's in your boat. The possibilities are :
> 
> Faulty wiring to the alternator
> Faulty alternator
> ...


That should narrow it down


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## TQA (Apr 4, 2009)

Assuming your alternator is OK and with the engine running is putting out current.

It is simple to check this with a voltmeter you should see 13volts +

Follow the heavy wire from the alternator to the batteries and look for a switch or more likely a diode or diodes perhaps fitted inside a thing called a battery combiner. if you have one of these you need the aid of soneone used to 12 volt electrics on a boat to test it.

SEE ONE HERE CLICKY

I would bypass it and take a wire from the alternator to the battery directly and see if that produces a charge at the battery.

ALL OF THE ABOVE IS DONE WHILE YOU ARE *NOT* CONNECTED TO SHOREPOWER OF COURSE!


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## motion300 (Feb 12, 2009)

My The motorola alt had the exciter wire connected to a oil pressure sw It would not charge until the oil pressure came up something check where is the exciter wire?


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## hellosailor (Apr 11, 2006)

Stu-
"the external regulator plug" Didn't mention any such thing. Some regulators will have a screw terminal for each wire, others have a plug for "everything but the output", which is usually a B+, lamp, and sense lead that are run in a common sub-harness. With separate leads and labels it is easier to find the excitor. Or, the terminal it was supposed to be attached to.<G>

From the oddball but possible department: If it is a "one wire" alternator, where the voltage sense lead is tied back to the output lead so only "one wire" goes out to the rest of the boat, as opposed to the usual three leads, some one-wire systems have to be goosed (i.e. to 1000 rpm for a minute) to get the alternator started. If you've been running low rpms...there's a remote chance that a 1-wire which hasn't been goosed just won't start up.

And fwiw, "modern" alternators with integral regulators designed in the last 40 years, should put out 13.8+ at idle, 14.3-14.4 at cruising speed, never higher or lower. An old design, or improper pulley sizing, might leave the idle output a little lower. And a cheap voltmeter may be off by 0.2-0.4 volts, misleading the user. But that's where the numbers "should" fall. Ignoring all the specifics, that alternator still just isn't working--on the boat. Taking it out to a shop for a bench test (again) isn't totally unreasonable, there are sometimes intermittent flaws where an alternator will test out perfectly good on one day, perfectly dead on the next. And too many shops that just don't somehow know how to test them properly.

"It would not charge until the oil pressure came up" A good way to ensure the engine isn't loaded up against the alternator until the engine is fully operational. Totally in violation of the KISS principle, of course.<G>

It certainly does sound like this will be one of those "A DUH!" real simple problems that is just real hard to spot. Something missing, disconnected, just not obvious until you know what to look for. "Where's Waldo?"


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## deniseO30 (Nov 27, 2006)

I had a similar problem.. it was just a bad connector on the start bank negative cable. (seems the PO never crimped it) no amount of charging ever "topped off" the 2 banks. 

Hellos, can you give me the dummies guide to installing an automotive 80-90 amp alt on my Universal 5416? (kubota based) I don't want to hi-jack this thread.


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## Stu Jackson (Jul 28, 2001)

TQA said:


> and look for a switch or more likely a diode or diodes perhaps fitted inside a thing called a battery combiner. if you have one of these you need the aid of soneone used to 12 volt electrics on a boat to test it.


That's simply not true. Combiners are relays, not diodes. In fact, that's the very combiner I have on my boat, has been working since 1998. No diodes.

hellossailor - in your earlier post you said: "Check for a plug with 2-3 wires in it coming off the alternator..." I must have misconstrued that to mean the plug in the back of newer alternators for external regulators. You could well have meant a wiring harness plug. Sorry about that. But since he says he has the same alternator that I know about and had on my boat, that's why I answered that way. No plugs, just wires, and wired as I linked to my sketches.

Denise - if you're simply replacing an OEM internally regulated alternator with another one, it's a simple swap. Just make note of the wires, as I did in the referenced link above, take the bolts off and remove the old alternator and install the new one. Make sure the power's off. If you are looking for a different electrical system design when you're doing this, or moving to external regulation, it becomes a different question, but it looks like you're doing a straight swap. Remember to put the belt on the pulleys before you insert the bolts to hold the alternator onto the engine.


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## paul323 (Mar 13, 2010)

Quick and simple test - buy a hydrometer (about $5) and check the batteries. I had a similar problem and it allowed me to find that one of the batteries was duff - a couple of cells were dead, and it threw everything out.

Then, as it turned out, one of the connectors on an earth wire just - er - fell off. So that probably didn't help. But the hydrometer allowed me to cheaply check the batteries.


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## hellosailor (Apr 11, 2006)

Hydrometers ARE nice, but a voltmeter never srips acid on your clothing. Let a battery sit for 24 hours after charging, test the voltage, and you've got the same "what's the state of charge?" answer. If it is shot, it won't hold a charge. And even with a hydrometer--supposedly the acid doesn't equalize for some hours after charging anyway, you still need to let it sit to get a really accurate result.


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## AdamLein (Nov 6, 2007)

A voltmeter can only tell you the surface charge on a battery. If there is a problem with individual cells, the only way to tell is a hydrometer.


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## hellosailor (Apr 11, 2006)

"A voltmeter can only tell you the surface charge on a battery"
I'd have to disagree, Adam. IF there's a surface charge, sure, that's what it will read. But you can also put a load on the battery (i.e. turn on some lights or motors) and read the voltage under load. And what do you think all the fancy battery monitor systems use? Hydrometers? No, voltmeters can tell you battery condition--if you use them properly. Simply sticking one on a freshly charged hot battery isn't the right way to do it. Put it on a load, get rid of the surface charge, and the voltmeter gives you reality, without the acid burns.


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## Titocummings (May 10, 2011)

*How would you know if your car battery is dead or just needs charging?*

my car won't start so i jump started it with another car. it worked. after a while, car won't start again so i had to do the whole process again. it worked. after about 20-30 minute of driving around, thinking that my alternator would charge it, i went back home and parked it and turned off the engine. waited a minute to start it again but it won't. so i cleaned the terminals with baking soda solution and tried to jump start it


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## efiste (Apr 5, 2000)

Thanks for all the replies. After checking out the alternator and batteries, it seemed I needed to focus on the wiring. I started by running a #10 temp connection directly from the alternator's positive (B+) to one of my batteries' positive terminal. Finally, the engine instrument's voltmeter showed charging life! Further evidence that the alternator and batteries are not the problem (ignoring wiring size and length possibilities). I skipped the more accurate multimeter testing for now.

When I called the yard mechanics that took care of the boat for the PO about the issue a couple weeks ago, I remember they mentioned that the PO had them replace the wiring harness as part of a tachometer replacement recently. The wiring indeed looks fairly new when inspected behind the engine compartment. When looking closely at the wire clip in the engine compartment (see picture in post), the top wire (orange; alternator output) has nothing connected to the other side of it. I am not 100% sure if the wire clip is a coupling device, hanger, or power distribution, but this does not look right. I think it is just a hanger or coupler. The wiring schematics I have seen for the Catlaina 34 in reference to researching this issue typically show the alternator output connected to the positive connection of the starter. I am thinking that the yard mechanics just did not hook up this correctly.

Thoughts, opinions, fruitful debate?

Eric


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## MarkSF (Feb 21, 2011)

That white block simply connects one wire to another - terminals on the opposite sides of the block are connected. It's a convenient but poor way of splicing wires together.

So that orange wire is definitely not connected to anything, and if the orange wire should be connected to something, that would be a problem. If it also happens to be the alternator output, I think you have found your problem. 

Congratulations are in order to the boatyard, for a job well done. 

Also, I wouldn't put 40-100A of alternator output through that block, no way. Can't believe it's rated for that kind of current.

So at this stage you have the choice of giving the boatyard that did it a kick up the arse, or just getting it put right yourself.


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## mitiempo (Sep 19, 2008)

I agree - get rid of the block and use at least 4 awg for B+ to the positive on the starter.


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## paul323 (Mar 13, 2010)

That white block tells me the person who did the work wasn't experienced with marine electrics. Not only is it a poor solution for a marine application, but as MarkSF said, no way should 40+A be put through that - alarming!

You should use a block like this 30 Ampere Terminal Blocks - Blue Sea Systems (depending on the amperage) with properly crimped connectors (see Marine Wire Termination Photo Gallery by Compass Marine at pbase.com for an awesome article). And frankly, you should probably check through/redo the wiring as the errors you have noted indicates more horrors may be lurking. That may sound daunting, but is actually not too bad - for a boat your size the wiring should be relatively straightforward, and it is the connections that really need the focus - but it is also very satisfying and you'll thoroughly understand your systems.

One more thing - from my experience - give any crimped connections a tug; if the right tool isn't used, they may just fall apart in your hands (several of mine did). And disconnect the 115V while working (if possible) - my PO had an unterminated live wire loose in a spaghetti tangle...could have been nasty.


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## hellosailor (Apr 11, 2006)

The block isn't the worst thing in the world, IIRC some of the Islanders and other reasonably well built boats used them to connect the engine harness to the boat's own wiring.

If you can, replace the block with something better. Ideally this would be a locking "pack connector" as used in auto harnesses--but no boatyard and few owners would want to pay for the time it takes to hand-wire one of those up custom. I'd redo this one, make sure there's some silion grease in the connections and that they are tight, and then BOLT IT DOWN to something.

I WOULD agree that putting the alternator output through one could easily exceed the rating, 40+ amps is a lot, and the connection is not the best. the alternator output lead should be a short, clean, heavy output if possible, and that's one place where using a heavier wire will result in less voltage drop, faster charging, less power taken from the engine by the alternator. A good place to use light cable rather than just wire.

The alternator output should also be FUSED and if there's no obvious point between that output and the starter post (which is a common place to run it) you might coinsider adding the fuse on whatever bulkhead or other solid mounting point is adjacent to that run.

Be disappointed in the yard, but not too upset. This is what happens when someone is continually interrupted or told to knock off at 5PM, and isn't doing the job with a checklist. Could have been worse but find the guy who did it, and tell him he owes you a case of beer.<G>


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## mitiempo (Sep 19, 2008)

I agree the block isn't the best solution in a boat, but the alternator output shouldn't go through a connector of any kind. I treat alternator output the same way I treat any other main battery cable. Heavier wire and direct to wherever your system has it going except for a fuse. Ideally the alternator output should go direct to a battery bank, commonly the house bank, with fusing near the battery. In this arrangement the start battery is charged by an Echo Charge or ACR. Catalina has for a long time wired the alternator output to the positive lug on the starter as it is connected to the common output on the 1/2/both/off switch with a fairly heavy wire, although the gauge they chose isn't as heavy as it should be. Previously they used a smaller gauge wire through an ammeter on the engine panel but most have upgraded this electrical bottleneck that limits charge output.


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## MarkSF (Feb 21, 2011)

"I agree the block isn't the best solution in a boat, but the alternator output shouldn't go through a connector of any kind. I treat alternator output the same way I treat any other main battery cable. Heavier wire and direct to wherever your system has it going except for a fuse. Ideally the alternator output should go direct to a battery bank, commonly the house bank, with fusing near the battery. In this arrangement the start battery is charged by an Echo Charge or ACR. Catalina has for a long time wired the alternator output to the positive lug on the starter as it is connected to the common output on the 1/2/both/off switch with a fairly heavy wire, although the gauge they chose isn't as heavy as it should be. Previously they used a smaller gauge wire through an ammeter on the engine panel but most have upgraded this electrical bottleneck that limits charge output.[/QUOTE]

The alternator output should go to the battery selector common, and however it's wired it should take full alternator output with minimal voltage drop - any voltage drop is potential charging that is lost!

I disagree about connecting straight to the battery, as then you lose the selector function. You may want to direct full alternator output to one battery bank that is flat.


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## mitiempo (Sep 19, 2008)

The popular way to wire charging circuits now is directly to one of the battery banks, house preferred. The start battery gets its charge either from an Echo Charge or ACR as I posted. The start battery is seldom down more than 1 or 2 AH as engine starting doesn't take much. 

There are several problems with the 1/2/both/off switch system most boats used to have the alternator output wired to - some sadly still do it this way. 
The switch has to be in the both position to charge both banks. If you forget and leave it in both you will drain both banks. If someone inadvertently switches it to or through off when the engine is running you run the risk of frying the diodes in the alternator. It does have one benefit for a builder - it is the cheapest way to charge 2 banks. 

The system I and many others like Maine Sail recommend is foolproof. It requires no switching for complete charging of both banks. And there is no way to do any damage even if the switch is in the off position.

There are many threads on Sailnet explaining its advantages.


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## Stu Jackson (Jul 28, 2001)

Going through the 1-2-B switch C post is not a good way to wire the alternator output.

OP - if you go back to my original post on this thread, it goes to my "Electrical Systems 101" topic, which includes a link to explaining that orange wire. It's part of the wiring harness upgrade that it appears has been done to your boat. Only YOU can tell us where the actual alternator output goes on your boat.


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## hellosailor (Apr 11, 2006)

"but the alternator output shouldn't go through a connector of any kind.... Ideally the alternator output should go direct to a battery bank, commonly the house bank, _with fusing near the battery_. "

See now, you've got to connect the output to the fuse, and the fuse to the battery, so now you've got two "connectors" in there no matter how you slice it. Whether the fuse is at the battery (as most are) or near the alternator, the number of connections are basically the same.

As best I can tell, the old style was to use a fusible link wire (which was called cheap and reliable but is now considered a fire hazard) from the alternator output to the hot lug on the starter, because this also used less wire and again, was cheaper. Why run from the alternator to the battery bank and then back to the starter, when you could daisy chain and save a buck? Of course seat belts were optional back then too.

As Chariman Mao said, "Black cat, white cat all same, catch mice."

Or as we hairy barbarians are more likely to remark, there are many ways to skin a cat. (I wonder if the black ones and white ones all _taste _the same?)


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## mitiempo (Sep 19, 2008)

"Why run from the alternator to the battery bank and then back to the starter, when you could daisy chain and save a buck?"

Because it puts the direction of charging on the operator and is not without its errors. Many I see leave the switch on "both" at all times - a recipe for the inability to start from either bank. 

The ACR or EchoCharge method, used by the better builders and in many upgrades, does the most to guarantee both banks are fully charged. 

As far as connections, a proper fuse in the alternator output has much better connections than that wire block in the picture posted. See below.


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## hellosailor (Apr 11, 2006)

Brian, you misread me. I'm saying, rhetorically, WHY would you run to the switch and put things under operator control AND INCREASE YOUR COSTS, when as a boat builder you can lower costs and increase sales, by daisy chaining the wiring instead?

The old fashioned fusible link and daisy chain WERE THE RIGHT WAY TO GO, if your priority was building a boat "to a price" and KISS.

We may not think that's validlogic now, but it certainly was accepted as valid then, and from the costs-only point of view, it still may be.

_Control _is a whole other issue. (G)


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## AdamLein (Nov 6, 2007)

hellosailor said:


> But you can also put a load on the battery (i.e. turn on some lights or motors) and read the voltage under load. And what do you think all the fancy battery monitor systems use?


Fair enough, I actually don't really understand testing under load. Should the voltmeter show 12.6 V for a fully charged battery under load? Or what?


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## mitiempo (Sep 19, 2008)

Depends on the load, but a battery can show a good voltage when charged and have no real capacity, in other words drop voltage considerably under a load.


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## hellosailor (Apr 11, 2006)

Adam, a brand new fully charged battery would show a nominal 12.6 volts (could easily be +- 0.1 volts or even .2 volts, depending on the meter and the battery chemistry) under no load, or under a small load, i.e. the memory circuit from a stereo, or some small LED lighting.

As you put on bigger loads (i.e. the starter motor on your engine) that battery might drop to 10 volts--and still be perfectly good. There's a relationship between the load and the battery and impedances and numbers you don't want to get involved with. If you REALLY want to test a battery under load, you can buy--or borrow--a _battery load tester _from many of the auto parts chains. Maybe $25 for a cheap one, $100+ for the fancy digital ones, but any of them would be "good enough" if it can be used with your battery. (If you have a 400AH battery bank, a cheap tester probabably can't handle it.)

Under a "reasonable" load, maybe the nav lights or the cabin lights or the instruments and stereo, I'd guess a new fully charged battery would drop from 12.6 volts to something like 12.3 or 12.4, then recover back up again when the load was removed.

Usually if you measure a battery 12-24 hours after it has been used and charged, when it has sat unused overnight, you'll get a fairly accurate number of the "real" condition. Which is something between 11.6 and 12.6 volts, every 1/10th of a volt being very roughly a 10% loss of capacity and anything under 12 really meaning "get a new one".


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## efiste (Apr 5, 2000)

*Issue Resolved*

Thanks for everyone's thoughts and assistance. I resolved my issue by connecting the positive (B+) output of the alternator to the positive post of the starter which is also where the batteries are connected through the navigation panel isolation switch. It is apparent that the yard mechanics did not fully understand how to complete the Seaward wiring harness upgrade.

I have also learned a lot more about my electrical system and look forward to upgrading the batteries, alternator, and wiring plan to more effectively maintain and manage my power system. Ah, next year... For now, I am looking forward to getting out and sailing!

Eric
1987 Catalina 34 (#358) - Windbreaker


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