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I have two 6V Victron AGM batteries (BAT406225080) wires in series. I recently had a failure on the circuit breaker for my battery charger, so my batteries weren't being charged and were run down to about 6V. Can anyone tell me bad that is for my batteries? I know it's not good, but is it more like "they likely need replacing" or "they'll be fine, just don't do that often"?
6V doesn't sound too bad for a 6V battery. It also depends, for how long the voltage was at that level and whether it has gone up a little after the loads have been stopped and it got charged again.
A single deep discharge isn't likely to have damaged the battery significantly, as long as it was recharged reasonably quickly (within a day or so). What destroys lead acid batteries is prolonged periods at a low state of charge as the plates get covered in hard deposits that cannot be removed.
You can undo temporary sulfation due to a deep cycle event by putting the battery through an equalisation cycle.
They are 6V batteries, but the 6V measurement I gave was the voltage across both batteries, so I assume the charge per 6V battery was more like 3V. I did notice the drop and start charging within about a day, so here's hoping.
@najaB, I'd love to know more about what an equalization cycle entails.
If you're using a Victron MPPT there will be an option to do a manual equalisation cycle in Victron Connect. Basically it consists of deliberately overcharging the battery for a number of hours at a higher than usual voltage with a restricted low current. This will dissolve any light coating of sulphur crystals back into the acid solution.
It only works with slightly sulfated batteries though.
