12 Volt Batteries Don’t Get Under 8 Volts on their Own

Two sets of batteries from municipal buses that we recovered from 2V
to successfully recovered at full voltage - 12.7V.
Batteries we recovered from 2V. In the battery business , we call such batteries “abused.”

The two sets of batteries in the photos came out of transit buses. The set of 4 batteries on the right all were at 2V when received. Fortunately, these are rugged batteries, designed to be rugged enough to withstand a deep discharge (this isn’t true of all starting batteries), and we were able to recover them.

12V vehicle batteries don’t get under 8V on their own. If your battery is under 8V, then it has been deeply discharged and charging for whatever reason wasn’t successful. You may have a charger issue, or used the battery in a way that it didn’t have time to successfully charge. If you attempt to warranty a battery under 8V, expect a competent battery shop to keep it at least overnight to charge it. Batteries can only be tested at full charge. If you have an appropriate battery charger, save all parties time and put the battery through a charge cycle before taking a battery with a nominal warranty claim to any battery shop. Given how much time it takes to recover deeply discharged batteries, don’t be surprised if you are charged for the service of recovering them. You get back functional batteries at a fraction of the price of replacing them.

How to recover deeply discharged batteries:

– Connect them either to a manual high current battery charger, or a high current smart charger with a constant voltage desulphating setting. I used a Genius G26000 26 amp charger. Leave them on this charger until they get to 11 volts. This is a high enough voltage a smart charger can properly detect and charge the battery. For these batteries, it took 12 to 36 hours per battery to recover them to 11 volts.

– Then connect the battery to an appropriate smart charger. In this case, the manufacturer specs a 40 amp charger with an AGM mode. It took another 6 to 12 hours of charging for each battery to bring them back to full voltage ~12.72V volts.

Also, “smart” battery testers aren’t always so smart. We have seen smart testers report bad cells when the batteries have only been deeply discharged.

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