Sounds like it's lost a cell or two. That's the usual sort of figure i would expect from one dud cell.
Disconnect (and insulate) the battery lead from the alternator and unplug the warning light/field wire so the alternator can't generate then try a fersh battery. It's possible if the alternator is dead and shorting the battery, it's what killed the battery or maybe even vice versa.
Once you get past these problems, you'll be glad you persevered. 😉 😄