Lead acid batteries work by depositing a chemical layer on the plates during charging, and changing it during discharge. Back when I was doing that kind of thing for C&G in tech, I could have spouted formulas with fair acuracy, but not following the exam. Because the powers of evil have so decided, the re-deposition does not always match and put stuff back where it was before, so it grows weak bits. Putting them to heavy use causes the plates to either warp or try to, which doesn't help, either.
Letting a battery self discharge lets subtly different chemical reactions take place, and while careful recharging "might" put life back in there, the plating will not be the same. A factory rebuild is needed. The good bit about lead batteries is that they are almost totally recycleable, almost everythng in them can be re-used.