12V Batteries Don’t Get Under 8V on Their Own

I have a set of four  high quality Series 31 AGM (Absorbed Glass Mat) bus batteries in my shop. They came in for warranty evaluation. All four batteries had a voltage of only 2V. Let’s cut to the punch line – 12V vehicle batteries don’t get under 8V on their own. The only failure mechanism to cause this to happen – two cells short simultaneously – is pretty unlikely. The battery manufacturer won’t even consider batteries for warranty if they are less than 8V, since the only likely ways to get below 8V can be described as one type of battery abuse or another.


The set of 4 batteries on the right all were at 2V when received.

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) How to recover deeply discharged batteries:

– Connecting 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.

Let’s get back to the punch line: 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.

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Jim Logan
Battery Giant of Pittsburgh
10 St. Francis Way, Suite 9
Cranberry Township, PA 16066

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