Is the battery bad or not?

I'm still having trouble out of my old IBM 380XD's battery. It will run fine for awhile, sustaining for about two hours or so from full to low, and then will go to a 5-minute runtime from full to dry. ...then after I cycle it umpteen times, it will run for two hours on a charge again. Anyone have any ideas as to what the deal is? I think it's actually something in my CMOS or something just not letting the battery charge, and after so many cycles, it realizes that it's not doing its job and recalibrates itself. Any ideas on what the deal really is and how to make that quit?