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Red Squirrel 08-21-2011 06:16 PM

charging multiple 12v lead acid batteries at once?
If I have 2 or more 12v batteries of same amp hour rating but with different voltage readings (ex: one is more depleted than the other) is it ok to charge them in parallel? Will they self balance or will the one with a higher voltage end up getting over charged?

Also is the proper way to connect to put the negative at one end of the bank, and the positive at the other end, or does it not matter?

micromind 08-21-2011 06:33 PM

Since the internal resistance of a lead-acid battery increases as it is charged, if two or more batteries are charged in parallel, the one with the least charge will hog more of the charge current until it reaches the charge level of the one with the most charge. Then, they'll all charge up together.

It doesn't matter which end the + and - load or charge is. If there's significant impedance in the tie cables, one or more batteries will charge and discharge faster than the others. Unless the charge/discharge rate is huge, the impedance of the tie cables can be ignored.


Red Squirrel 08-21-2011 07:15 PM

Good to know, so basically I'm ok then.

I've been experimenting with some old UPS batteries, and noticed that the one that is topped up is warm while the other is cool, is this also normal? Now it's also possible these batteries are bad, they are old.

dmxtothemax 08-21-2011 08:19 PM

If they are all lead acid batteries,
Then the batteries internal impedance will regulate the charge,
If a battery is getting hot, that indicates a faulty battery,
Or overcharging.
Cause if the battery is ok then it regulates its own charge.
But if the battery is faulty this might not happen, so it gets hot.

I am sure its not the technically ideal way to do it,
But it does work most times.

micromind 08-21-2011 10:04 PM

Usually, an abnormally warm battery indicates a shorted cell. Not always though.

To test, put some charge into the battery, then let it sit disconnected for a couple of hours. If the voltage is around 10 - 11.5, there is a shorted cell. If it's over 12, then all cells are ok.

A battery will also overheat if the charge rate is too high, or if it is being overcharged. The warmer it is, the lower the float charge voltage should be.

If the float charge voltage is kept too high, the battery will go into thermal runaway.


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