[Shotimes] Charging Problem

[Frank & Susan Malinowski]

I have some kind of electrical problem but it is not obvious to a
simple analysis.  I recently changed out my battery for a new one at
Sears. My old one was several years old and had almost left me
stranded. The mechanic advised me that the alternator had a problem and
I should have it serviced. Since I have under drive pulleys I was not
too surprised but I went on to have it checked elsewhere and it seemed
not to be working correctly and I had it changed out with an rebuilt.
This one soon proved defective and the shop replaced it at no cost but
now the battery bubbled a bit which I assume is caused by over
charging. Sears replaced the battery no charge :) but a week or so
later the battery is bulging again. I had the charging system checked
out and the mechanic said it is all fine, the alternator is putting out
the correct voltage to the battery and the battery is correct. Nothing
in the normal system is out of order.
So I wonder if there is some problem that randomly shows up and causes
this kind of problem. Any suggestions on what I should look at to
determine why these batteries are bulging from, I assume, over
charging?

I have a 1992 MTX 150K miles, under-drive pulleys.