One charger at a time…

Two 6-volt batteries in series only need one 12-volt battery charger (most of the time)

Good morning Mike,

My wife and I very much enjoyed your seminar at the Hershey Rv show this year. I want to charge my two 6-volt batteries that are connected in series.

So, I plug my 30amp cord into shore power. Then I plug in two battery chargers into an outlet within my interior space under the RV to charge my two gel batteries connected in series. The 3A charger is connected to one battery and the 12A charger to the other.  

The Delco dealer told me that the small charger would take “days” to charge the batteries and said I would be better of using the 12A charger. 

The more the merrier I say and connected both. The 3A this last time around noted “aborted, bad battery” !

I hope this does not apply to me (stupid is what stupid does) - Dan T

Hey Dan,

These are 6-volt batteries connected like this in series, correct?

If so, you should only connect the 12-amp charger between the negative terminal of the grounded battery (shown on the left) and the positive terminal of the battery feeding you RV load center (shown on the right).

I don’t think you hurt anything, but you don’t want to connect more than one charger at a time.

Charging time

A pair of 6-volt, 225 Ah batteries in series creates a 12-volt, 225 Ah battery. To figure out how long that should take to charge, just divide the Amp-Hrs of the battery by the amps to calculate the hours. So 225 Ah / 12 Amps = 18.75 hours. Now, no charging is 100% efficient, so let’s round that up to 24 hours, just to be safe.

If you’re recharging from 50% to 100% SoC (State of Charge) it should only take 1/2 of that time.

So my SWAG is that your 12-amp charger should be able to recharge your two batteries from 0% to 100% SoC in about 24 hours, or from 50% to 100% SoC in about 12 hours.

When to charge one battery at a time…

If your charger can be set for 6-volt batteries, then this trick could be useful. If you do run your batteries down flat (close to 0 Volts) then it’s better to recharge them one at a time with the charger set to 6-Volts, 12-Amps. This can help get equal charges on each battery. It should take around 24 hours to charge each battery separately, so 2 days total.

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