One of the things that happens to a battery in normal use is the plate oxidizes and eventually there is so much crap on the plate it does not conduct electricity well. Many things can go wrong, but the most common is plate oxidation in a normal situation. Heat is the cause of most battery failures. I have run in to reversed battery hook-ups with 8Ds in trucks and buses at least once or twice a year. I suppose it is because of how the battery looks was my best guess. Often the tiny little plus and minus are hard to see on the 8Ds. Mind you this is just a guess as I can't see how one could make that mistake, but I suppose it could happen if one was having an LSD flashback. I have charged batteries backwards to remove this deposit. It works on some batteries, some blow up because of an internal short. Oh well, I still think it is worth a try if one has an area that will not screw up in case of the latter. How about running a couple of hundred volts AC in to bad nicad packs for just a second to blow the crap off of the plate. It works too, but sometimes the results are the same as the lead acid. BOOM! Just remember, don't try this at home! The newer sealed batteries loose their electrolytes after a while and go south pretty fast.
Regards,
badmoon