You're going to need to recovery charge this battery. Yes, it means overcharge to the 17 volt area, which means popping the vents in the process. Outdoors, in a clear area.
Refer to the Lifeline manual for a full run through of this process. It does not mean losing an appreciable quantity of electrolyte.
But results often bring forth magic if the electrolyte did not skin the plates during the overly long hibernation.
Follow the Lifeline manual to the letter. And allow 6 hours for cooldown and vapor purging.
I would heartily recommend wrap around safety goggles, AND a face mask. Gauntlet gloves and a cap of some kind to protect scalp.
Let your instruments and the manual do the thinking. Complete the task then after cooldown resume your tinkering. Sometimes this exercise needs to be repeated so don't be shocked if the first time is not a charm. Let 3 days elapse between events.
