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Old 09-25-2002
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Charging Batteries

I have a question about charging a starting battery and a house battery with different Ah capacities. What is the best way to charge them both together using one alternator? I have heard that using an isolator will cause a voltage drop in the charging voltage, thus causing a slow charging process. Also, if the two batteries being charged have different capacities and are at different states of charge, will charging them together cause one to be overcharged while the other is undercharged? So what is the most ideal way in seting up a charging system for such a battery setup. And what would be a compromise setup in the case of a low budget, without shortening battery life.

Sue & Larry respond:
Thanks for the question. You correctly point out that using a battery isolator results in charging inefficiencies due to a voltage drop. With an isolator, the voltage drop can be as great as 0.7 volts.

One way to address your two-bank charging concern is to install an alternator that is configured with a dual output. Dual-output alternators are able to sense the charge state of each battery or battery bank individually. Charging current is then provided to one or both battery banks only when needed through separate charging circuits.

In the absence of a dual-output alternator, a two bank Battery Combiner could be installed. A Battery Combiner automatically connects two battery banks together when charging current is present. When you start your engine and your alternator kicks the voltage up to say 13.5 volts, the Battery Combiner electrically joins the two banks. When your engine stops, the charging will stop and the voltage will soon drop. This drop in voltage is sensed by the battery combiner, and it separates the two banks so that they will both not be discharged. You will only continue to discharge from the battery bank indicated by the battery switch.

The normal low-budget method of charging a two-battery bank on a sailboat is to switch your battery switch to "Both" while running the engine, then back to your house bank once youíve stopped. If you forget and leave the switch on "Both," then you will discharge both of your battery banks and may not have any power left for cranking. This is a mistake that is very easy to make. Adjusting the battery switch with the engine running can also be a little tricky. You can move the switch from "1" to "2," or "2" to "Both" as long as you move the switch quickly. If you pass by or stop the switch on "OFF" with the engine running, you risk blowing the diode in your alternator. There are special switches that eliminate this risk, but theyíre not very commonly seen.

Thereís a final low-budget and simple-minded approach that we like and have used ourselves for the last six years while cruising and living aboard. We consider battery bank No. 1 to be our main bank. We use it for both the cranking of our engine and also for all of our house electrical needs. We almost never turn our battery switch from bank No. 1.

Battery bank No. 2 in our mind is our emergency battery. Itís comprised of a single group 31 cranking battery. Every few months we top off the charge of this battery if necessary. It generally holds a good charge for four to five months. Itís comforting to us to know that we have a fully charged cranking battery ready and waiting to go if we goof up and deplete our main No. 1 bank.

When we start our engine, we donít have to worry about battery switches and such. We simply start our engine with our main bank and recharge this bank while the engine is running. Itís all very simple.

Best of luck

 

 

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