I have a AGM deep-cycle battery, 150 Ah, 12V: Banner "Stand By Bull" SBV12-150. 12V150AhC20.

Up until recently it was working quite OK, I was able many times, for many months, to drain about 170 mA from the battery for a month before recharging it (such a drain should not deplete the battery below 20% full).

But recently the performance suddenly dropped, the last time I tried to charge it, here is what happens: the voltage on the battery without any load before starting to charge was 9.69 V. I don't know why it was so low, because the battery was charged previously only 10 days ago with 16 hours of about 5A charge, at the end of which the voltage on the battery (while still charged) was 14.7 V, and the specifications of CCCV charging of an AGM battery say that an AGM battery is at least 80% full when the voltage reaches 14.1 V, so 14.7 V should be really full. And then the battery was used for 10 days with a load of about 170 mA, so no more then 41 Ah should have been depleted from the battery.

This time (after these 10 days of 170 mA load), I did 20 hours of 2A charge (which should put about 33 Ah back into the battery). The voltage on the battery at the end of charging was 14.0V. Then I disconnected the charger, and the voltage measured on the battery just after disconnecting the charger was 13.0V.

And then I placed a 60 mA load on the battery, which is a very tiny load for such a big battery, and it should go for about 40 days like this on an 80% full AGM deep-cycle battery before the voltage should drop below 12V. But in my case, the voltage dropped below 12V after only 10 hours - this is about 100 times less than what it should be.

Is there anything I can do to try to revive this battery? Is there something like a desulfation or equalization charge that I could try to restore this battery, and how to do it to have the highest chance of success?


I found an article on deep-cycle batteries, and they comment that if a battery goes below 10.5V, it's "dead" due to the specific gravity of the acid being too low to be useful any more.

It appears that you may need a new battery. : (


A battery with 12.7 volts is fully charged, 12.5 volts is 90% charged. If the battery drops below 10.5 volts after the floating surface charge is removed (wait three hours after disconnecting charger), you have a shorted out cell (electric short between plates). You can remove the caps and measure each cell's voltage with a tester.

Shorted out cells in sealed case batteries cannot be repaired. Replace the battery.

I do hundreds of deep cycle gel and AGM batteries a year. I use $14,000 battery regenerators / desulfators. Most electronic charger/desulfators are a scam. A good machine warms up the battery in the process because of the voltage and current required. Little 'advanced electronic chargers' costing a hundred dollars aren't worth their claims. Buy a Schumacher SC8 from Walmart for $80! It has an automatic desulfator cycle when necessary for almost dead batteries.

AGM (AcidGlassMat) batteries are 'dry', with all the electrolyte absorbed into the mat, but may have dried out the mat too much from overcharging. Adding 1 ounce of distilled water to each cell may help. Do not fill with water or electrolyte above what can be readily absorbed by the mat.

  • \$\begingroup\$ AGM batteries are sealed by definition. Suggesting someone add water or electrolyte makes the quality of this answer rather suspect. \$\endgroup\$ – patricktokeeffe Apr 21 '17 at 19:50

If you can get the battery to 13V 24 hours after the charger has been disconnected, no cell are probably dead.

I would use a desulphator for several months on it as it seems to be a large battery and the rule of thumb for the desulphating time is one day per pound of battery weight.

  • \$\begingroup\$ After doing some searching, this appears to be a questionable technology. \$\endgroup\$ – gbarry Dec 12 '12 at 20:17
  • \$\begingroup\$ I have tried it with several batteries and it does usually work. I would recommend to read about it here: leadacidbatterydesulfation.yuku.com \$\endgroup\$ – Gunnish Dec 12 '12 at 20:20

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