Fixing a Pb AGM battery with 1 bad cell

Got an RBC7 type UPS battery on sale. It consists of two 12 volt 17Ah AGM batteries. One is ok, nearly new, but the other has one bad cell. It loses voltages of this bad cell and pops the cap when discharging (gassing when reverse charged).

Also interesting behavior is that it's internal resistance rises slowly, but steadily into whole ohms territory when left like this - partially discharged - but when load or charge is applied, the resistance immediately drops to like 40 miliohms.

Is there any easy fix - like adding distilled water or acid to the bad cell?

Please no fears, I know what I'm doing.
Thanks

Hello Milan,

I have not been able to find a way to restore those type batteries. Usually when they go bad it is time to recycle them. Keep in mind that spill proof batteries usually won't respond well to adding liquids to them.

Tom

You can also find bare SLA or AGM cells from various battery manufacturers for much cheaper than buying the "pack" from APC. Keep the connecting wires and metal trays, though

Replacing it is your best bet.

However, there is one possible way to recover it (not fully due to the abuse to follow), is to discharge the entire battery to practically nothing. With an adjustable bench supply, set the current to no more than .001C, without applying too much voltage, which in this case would be about 18ma. Let it charge at this rate for many hours. Raise the voltage a tiny amount to increase the current, perhaps doubling it to 36ma. Let many more hours pass. Continue doubling like this until you reach a normal charge voltage that your regular charger can handle, typically 10v or more.

For all this work, you'll end up with a very abused battery, but in an emergency, I suppose .... Also consider that there is more damage than just reverse charge if you don't get to it in time.

About watering an agm by popping the caps. When you add distilled water to an agm, you are changing it chemically from an absorbed-glass-mat, to a poorly-performing flooded battery. Now it has to follow the rules for FLA, and not agm. That means very high self-discharge, and very low current handling capability. For those that do this, they often make the mistake of not lowering the charge voltage like you would for a flooded. In this case, I would never charge these agm/flooded frankenstein goofs higher than 14v.

Since your time is worth something, just replace these things.