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Using Larger 12V Batteries to charge smaller 12V Batteries.

Using Larger 12V Batteries to charge smaller 12V Batteries.

Old 04-28-14, 02:43 AM
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Using Larger 12V Batteries to charge smaller 12V Batteries.

My question is thus, I have a small solar power setup. The batteries are deep cycle 60 ah three in series. No issue there.

Often times we land up in a situation due to main power going out (we are plagued with cable theft issues where I live). We need to top up some 7 ah or a 35 ah batteries that due to length the outage, need to be charged to give us light etc.

Normally, I would use a battery charger to recharge the batteries at the end of an outage and not bother with all this. But during long power outages I can't do that.

During a recent outage it occurred to me I have all this 'power' sitting just there.

To charge these little batteries can I just connect them and charge them. I am afraid to do that in case the three 60 ah batteries effectively 180 ah would 'over whelm', the little 7 ah Battery's. Cause a fire or some other mishap while my back is turned.

Another thought I had was to get a cheepy charge controller and put that between the deep cycle batteries and the 7 ah batteries to they donít get overloaded.

I am wondering what you guys would do in this situation.
Old 04-28-14, 03:13 AM
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Welcome to the forums. It would be like trying to fill an empty glass with a glass of water and trying to keep them both full. In your case I would opt for a solar charging system
Old 04-28-14, 04:51 AM
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The charged (here, large) battery will charge the discharged (here, small) battery, but with two caveats:

1. Yes, the large battery may send power so rapidly to overheat the small battery, Chargers have current limiting circuits and should be rated for the subject battery. You could construct a current limiting circuit but you would not want to because that would waste precious energy of the large battery during the charge cycle.

2. The small battery will not be charged all the way; you need more than 12 volts to top off with. In reality the large battery's voltage will have dropped a little below 12 volts well into the charging cycle when that battery has given up much of its charge. Charging stops when both batteries are at the same voltage..

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