lead acid batteries have one nasty characteristic: when you use them (draw them down) you have to recharge them immediately
and I mean immediately.
That is how they work in a vehicle, you start the engine and take a lot of power out, and the alternator immediately charges the battery back up.
If a battery slowly goes dead over days or weeks, its usually shot after that. This is why you put a trickle charger (keeper) on your MC battery over the winter. Normally a battery will just stay charged for a few months by itself, but if its slowing draining down and you dont keep it charged, the chemistry on the lead plates turns into something else, and the battery will not recover.
Some people say you can put something in there (I dont remember what) and recover the battery after its been dead for a long time, but I dont know if that is true.