Could be a bad battery, could be a bad (regulator/converter). I would connect it back up (put on a pair of goggles and have a water hose nearby) and measure the voltage at the battery. Measure when not charging and measure when charging. When not charging you should see somewhere around 12.7-13 volts with no load. Then measure again when connected. You should see something around 13.2 to 13.8 in warm weather. The problem is the battery may already be destroyed and the converter may have done it, so both could be bad.
Jim and Katy
w/ Shelby and Sealy (Bernese Mountain Dog and a Nice Mut)
HAM CALLSIGN - AI4SR
EE, IBMer, Race Car Builder, now Retired.