Minimum battery voltage for onboard charge

I have owned a bone-stock 2009 eL since new. The AGM batteries are less than a year old. I made the mistake of leaving the main contactor and the ignition switch both ON while I was gone for three weeks. The batteries are down to around 5 volts each and the on-board charger obviously will not charge them.

What’s the minimum voltage at each battery for the on-board charger to take over?

Thanks much,


DeltaQ says 1v per cell.

Thank you sir! Each battery is currently at around 5v. Pardon my stupidity, but how many cells are there in a 12v battery? FWIW, I assume 6.

Never tried it, but someone suggested adding an extra 12v battery to the string, until voltage comes up.
Use a battery that will accept a charge or a junker.

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Could be a very good idea, but I have no 12v batteries sitting around at the moment.

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Take it out of your car.

If my car were still a '55 chevy, that would be simple enough to do. But with my BMW …

Lawn mower, motorcycle? Steal one from a neighbor? :stuck_out_tongue_winking_eye:

One hour charge on one battery brought the charge up to 11+v. Tomorrow I’ll use two chargers and see if my problem is solved without breaking into my neighbor’s garage.

When you get done boosting, and the charger kicks in, let them all go through a charge cycle(which is obvious). Then let them cool off a bit, then give them another cycle.

After they cool off again, check each one with your meter again. Hopefully they will all be fairly close to each other. Individually boosting messes with the balance and sometimes you might need to bring up the low ones with your charger.


Thanks. I’ll do that.

I brought each battery up to around 11v, then plugged in the charger. It charged for a bit, then got two flashing red lights. Unplugged and tried again. Charged a bit and four flashing red lights. By this time the dashboard was illuminated, the SOC indicated a full charge and the voltmeter indicated 72+ volts for the pack. BUT, when I applied the throttle, I got an error 15. It seems the car REALLY wants me to follow the “alternate charge” method in the owner’s manual. Soo – turn the main disconnect off and on and try again. Again the four flashing lights but the car will move now. So I drove it around the block and plugged it back in. Got a full green light charge and all is now well.
I will NOT forget and leave the key on the next time I leave for three weeks.
Thanks for all your help,

But don’t go away just yet. I don’t think you are done. Your battery pack needs a full checkup. Do a V check on each battery and make sure they are within reasonable balance. A full drain like this might have stressed one or two.

Do you know how to do a brake stand test?

Several days and several miles after a “green light” charge, all batteries measure 12.56±.02 volts.

I know how to do a brake stand test, but have yet to do so.

Thanks again for your help and concern,

Charge the batteries individually, after they are all up to 12 volts, then connect them together…