You have a "sulfated" battery- - - -one where the normal chemical process of charging and discharging has been interrupted by the formation of lead sulfate crystals that reduce the current capacity without affecting the open circuit voltage. Loaded voltage will drop dramatically. I taught battery theory and servicing for 30+ years. There's a way to break down sulfation, by completely discharging the battery and S L O W L Y recharging it repeatedly, (10 amps or less) but it takes weeks and numerous cycles to work. Very few people have the time or patience to do that.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!