6v agm batts voltage difference

Gkopan · April 7

After wintering (in WI) my batteries together we're reading something over 10 volts. After (only solar) charging, they are now ready 13.88v, but one reads 7.07v and the other 6.88. is that difference a problem? (FYI, We are leaving for 3 wks on 4/21 with several nights w/o hookups.) Thanks!

Grumpy_G · April 7

The difference is only 0.19 V, not enough to worry about. However I'd plug the camper in for a couple of days so the batteries see a constant voltage. With solar the batteries do not have constant charging and do never get a proper absorption charge.

AnOldUR · April 7

If you have access to a 6V charger, you could fully charge each one separately and then recheck the voltage. As @Grumpy_G said, there's not enough difference to worry. They should balance once charged at 12V in series.

Gkopan · April 8

Thanks much to each of you. I'll plug it in!

pthomas745 · April 8

Below is a 6V battery "state of charge chart". Keep in mind: if these readings are all from batteries that are being charged, the voltage will be showing artificially "higher than actual state of charge." The only way to get a reasonably accurate voltage from a lead acid battery is with the battery switch off, and no charging coming in.

And, apparently you have a multimeter and know how to use it!

Image: https://us.v-cdn.net/5021717/uploads/editor/ob/c1g5cix69pgd.jpg

Gkopan · April 14

Friends, My two six volt batteries are still different after charging on shore power with the battery switch off for a couple days. One is 7.04 the other 6.66, so the gap is a little larger now. When is it a problem to have a gap, how much, and what can happen? We have a 3 week trip coming next Tuesday.

Grumpy_G · April 14

I'm confused, why would you have the battery switch off when trying to charge ? Unless the trailer was modified that disconnects the converter/charger from the batteries.

Gkopan · April 14

OK, thanks. I'll try again, shore power on and batt switch on.

Gkopan · April 14

In any case, can you tell me what is too big of a gap between the two batteries? Thanks.

AnOldUR · April 14

Keeping the difference under 0.1V is recommended.

Gkopan · April 15

Thanks. I'll ck it again after a couple more days of charging.

subkron · April 15

The cell voltage measurement should be taken a bit of time after the charging source is removed and no loads are being pulled. The most accurate would be to disconnect the battery via switch and either measure when you have no solar gain or shut off solar charging via the Victron app. There still will be about 20mA pulled from the MPPT but that should not make much difference. Of course give it a bit of time after all that to let everything reach steady state.

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