Not exactly automottive, but I have a large genset that is started using two 12V batteries connected in series. For the most part each battery had been kept in a charged state with its own smart charger. But there is one of the old batteries (that had been starting the genny) sitting nearby, and occasionally I would disconnect one of the chargers and reconnect it to the old battery to keep it in a relatively charged state as well. Forgot about this for several months, so the one smart charger charging the one battery apparently was charging the other battery in reverse at the same time, such that I find that battery has now reversed voltage to -10.3V. Am now wondering if when both smart chargers were connected, were they to a certain degree also working at cross purposes i.e. also trying to charge the other battery in reverse? Could this explain a shortened battery life? Am also assuming an appropriate diode or two could resolve the issue. Has anyone had any experience like this, or the appropriate knowledge to suggest a good resolution to this issue?

 

Put a load on the now reversed battery and run it flat dead then charge it again the right way.

After that just pay more attention to how you have the related charging system set up.

The now reversed battery might show a considerable reduction in its capacity and service life for its adventure but it will likely recover well enough to be useful.

 

I haven't seen that one for a long time but depending on how old the batteries are, I would try to get a sense on how matched they are now as far as capacity. I would think there will be a negative effect on charging the good battery now and the reversed one will have a much shorter life expectancy than before. When you replace them, replace them both at the same time.

 

I haven't seen that one for a long time but depending on how old the batteries are, I would try to get a sense on how matched they are now as far as capacity. I would think there will be a negative effect on charging the good battery now and the reversed one will have a much shorter life expectancy than before. When you replace them, replace them both at the same time.

Great, thanks for that info. But I realized probably the easiest things to do now, now that I have replaced the reversed battery, is to simply use a 24V charger. These batteries were probably only a couple years old, and the other battery is still showing over 13V, so it should be fine. The reversed battery I think I'll just keep reversed, and it should still be usable for emergency lighting, using LED's. Have a great need for longer lasting emergency lighting anyway.