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FishFinder 08-30-2008 04:38 PM

Battery Selector: Run with ALL or 1 or 2??
Well I just replaced my 28 year old alternator (on a 1980 Volvo MD7A) which has an internal regulator ... $360 bucks! All seems to be well, getting 14.4 volts at the battery when running at moderate RPMS. I want to be sure I don't fry this one.

I've never had a functioning alternator on a boat, and here is my setup: battery 1 is an engine starting battery, battery 2 is a higher-rated "house" battery. Both are 12v. My battery switch (also from 1980) is supposed to be a "make before break" type switch, but I have noticed sometimes when switching from bat 1 to 2 my electronics turn off. Hmm. And I have tested the alternator output, and only the battery selected by the battery switch receives a charge when the engine is running.

So my question is, how should I set the battery switch on my boat when starting and running the engine?? Should I trust my battery selector and switch between batteries while running? Would having it on "ALL" for starting/running be OK even if my 2 batteries are at different charge levels?

Thanks. My boat is a 1980 S2 9.2A.

JimsCAL 08-30-2008 05:07 PM

With just a basic battery switch, you want BOTH on when running the engine to keep both batteries up. I would be concerned that the switch may have issues if the electronics shuts down when switching batteries (I assume you are not going through OFF but through BOTH. Until I had that issue resolved, I would start the engine on BOTH, leave it there until stopping the engine, then shift to the house battery. That should prevent frying the diodes in the alternator.

hellosailor 08-30-2008 05:14 PM

A 30-year old battery switch may not be making proper contact anymore, the contacts do wear and sometimes shift. I'd be tempted to replace it, or to install a battery combiner switch (West/Yadina or Hellroaring) which starts you on the starting battery, then combines both to charge/run once the alternator has come up to full voltage in a minute or two. When you shut the engine, it disconnects them again, so your starting battery is just isolated for starting use while you are on the house bank.

gc1111 08-30-2008 07:37 PM

No matter what it says on the switch or how new it is, idle the engine before switching. It is rare to blow the diodes by switching at idle, even with an old fashioned switch. (Rare does not mean never though)

camaraderie 08-30-2008 08:21 PM

I ALWAYS use either battery one or two but not both when running the engine. Should you lose your belt or alternator output while will drain the battery the switch is turned to...making it impossible to restart the engine after repairs. If you have the switch at wil drain all. Ask me how I know! :D
My suggestion would be to charge your batteries at the dock and start and run on your house bank. Get an echo charger for your start battery but otherwise hold it in reserve for emergencies.

Maine Sail 08-30-2008 09:18 PM


Originally Posted by gc1111 (Post 360899)
No matter what it says on the switch or how new it is, idle the engine before switching. It is rare to blow the diodes by switching at idle, even with an old fashioned switch. (Rare does not mean never though)

At idle, depending upon bank depletion, my alternar can put out well over 40 amps. Even a 3-5 amp output is enough to fry diodes! Idling the engine will do NOTHING to save your diodes from being fried if your bank was low when you started the engine.

I run a solar panel so my batts are always charged. My brother accidentally switched the battery switch off, before I shut down the motor, thinking he was helping and it fried the diodes even though the alt was only putting out about 3 amps at the time into a 99+% charged bank...

DO NOT use your "start" battery to start your engine use it as an "emergency" battery and you'll be fine if you add an ACR Automatic Charging Relay. Automatic charging relays only combine the banks when they sense voltages over about 13+ volts and they automatically disconnect at about the same..

In the last 15 years or so I have only once started my engine using my emergency bank and that was due to a stuck bilge float switch that killed my house bank..

Both my house and my start/emergency banks are deep cycle batts so that I'm combining batteries of like age type and size when charging. Contrary to popular belief you do not need a "start" type battery to start a sailboat aux engine..

If your batt switch is breaking before making buy a new one!!

gc1111 08-31-2008 02:34 PM


Originally Posted by otaga05 (Post 361084)
I would suggest that you charge them separately, especially if they are different types of batteries as is often the case. Even if they are the same type, the two banks will often be in different charge states and require different voltage and current regulation for optimal charging. Eg, the house may be deeply discharged and require a different voltage than the starter which is close to float levels.

I disagree. What is optimal for each battery separately is not optimal for the system consisting of both. Think of them as a single bank consisting of paralleled batteries. The charging current will divide between them according to their needs.

btrayfors 08-31-2008 02:36 PM


Good advice. Charge them separately or, better, use a Xantrex EchoCharge or a Balmar DuoCharge device (NOT a battery combiner or an isolator, etc.).

Sorry, halekai36, while I very much value your expertise and thoughts on many things, I have to disagree on your philosophy re: starting batteries.

The preferred setup is:

1. ONE large house bank, charged by every onboard device (battery charger, alternator, solar, wind, generator, etc.);

2. ONE starting battery, sized for your engine and NOT a deep-cycle battery; and

3. an EchoCharge or DuoCharge device, as per above.

This gives you a near bullet-proof, very efficient setup, and provides all the safety and redundancy you need.

Switches, fuses, breakers, etc. are a related but separate topic.


gc1111 08-31-2008 02:51 PM

Another good design is to have an automotive alternator charging the starting battery and a separate, high output alternator with a 3-stage (or more) regulator/monitor system for the presumably much larger house bank.

Maine Sail 08-31-2008 04:37 PM

Guys please read the OP. This guy just replaced his factory alt with another factory alternator (no smart regulation). He is most likely NOT going to spring for a $600.00-$1400.00 charging circuit upgrade based on his original post.

He can buy a cheap combiner and a new battery switch for less than $125.00 and be on his way and doing fine for short change. Dual alts or a Balmar configuration is certainly not in his cards having just spent $360.00 on a factory alt and being shocked about the cost of it.. I certainly don't get the feeling that he is a candidate for a gourmet charging circuit designed for long term cruising use..

"Well I just replaced my 28 year old alternator (on a 1980 Volvo MD7A) which has an internal regulator ... $360 bucks!"

ACR Charging Relay / Defender $79.99

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