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?