Actually, Battery Charging Question

OK, I bought a 24v BatteryMinder charger for the purpose of keeping my 24v military vehicle battery system maintained(2-12v Optima Red Tops connected in series). I thought I understood the store tech staff to advise that I disconnect the 12v batteries and “qualify” them before connecting them. I did so, and at rest, one battery read 12.2v and the other read 12.3v on digital voltmeter. I then connected the 12.2v battery to the 24v battery charger. The charger immediately indicated that the battery was weak or sulphated but did not reject it. So, I then connected the 24v charger to AC and left it to charge for 6-8hrs. Somewhere during that time, the charger rejected(timed-out)the battery and would not enter into float mode. Again, I thought I understood the tech staff and the manual to state that I should repeat this process again. I did so, and with the same exact results. So, I disconnected the battery, let it rest about 12hrs and tested it with digital voltmeter. Voltage read 12.5v.

I then repeated the exact same procedure with the other 12v battery and got the exact same results. Rested voltage on this battery was 12.4v.

I then decided to connect the two 12v batteries together in series and connect the BatteryMinder 24v charger. All seemed fine at first. Battery condition immediately read good(which surprised me), but after about 5 minutes, the battery condition indicator lit up indicating a weak or sulphated battery and immediately entered into float mode(also surprised me). Anyway, the manual clearly states to leave it at that mode for no less than 72hrs. But, after 3hrs, I decided to check the charging voltage w/digital voltmeter(24v charger still connected) to each 12v battery thinking both 12v batteries should have same readings. Nope!!! One battery read 12.8v and the other read 14.2v. SOMETHING’S WRONG ISN’T IT??? Have I damaged these expensive Optima Red Top Batteries? Please tell me I haven’t done that. A (mechanic friend)tells me to take both 12v batteries and have at least a 40amp power charge put on each one of them for about an hour. Does that sound right?? Please, any and all info would sure help me sleep better.

I take this military truck(pic in profile)for several days at a time, where only a goat can go, and I’m way to old to be hoofing it out some 10+ miles to civilization just because I was to ignorant to maintain batteries. So again, any and all help, info would be greatly appreciated.

Electronically Challenged,


OK. First, you bought the wrong charger for the first scenario. If you were going to remove the batteries, and charge them separately on the bench, then you should have purchased 2 12V minders. You should NEVER hook a 12V battery to a 24V charger. It will overcharge and damage the battery. The only way this will work is hooking them to series, and with the charger. When you checked the batteries again, did you disconnect them apart first? I still believe that 2 12V minders would be best.

I agree with Busted. Don’t try to use one 24V charger to charge two 12 Volt batteries and certainly not one at a time.

Doesn’t your 24v charger have a switch to output either 12v or 24v? I suspect you had the switch in the wrong setting at the wrong time.

Agree. 12 volts is 12 volts. This is similar to going overseas and trying your 120 volt electric applainace on 240 volts! Nasty things can result.

There is no reason that you should not be able to charge 2 matched, 12-volt batteries in series. In this case, you have 12 nominally 2-volt cells in series. In a 24-volt battery, you have 12 nominally 2-volt cells in series. 12 of one or a dozen of the other, I say. To say that it would be better to charge them separately with 2, 12-volt chargers is true. It would also be better to charge the individual cells with 12, 2-volt chargers.

To the OP, you might have damaged the batteries. It really depends on the charger. These are AGM (adsorbed glass mat) batteries. Does your charger say it works for those types?

To charge two 12 volt batteries with a 24 volt charger, the batteries must be connected negative post to positive post. This is series, just as in your 2-cell flashlight. The charger connects to the unconnected negative post on one battery, and the unconnected positive post on the other battery.

How much time do you have vis-a-vis this problem? I would like to help you determine if your batteries have been damaged. Please post back w/ battery model #, any and all specs, info CCA RATING ESP. For now err on safe side and DON’T DO THE 40 AMP CHARGE SCENARIO. That may have been a way to knock sulfate off plates on the old vent cap style lead acid batteries. Please post back w/all this info. Hope you’re not in a huge hurry.