Pontiac Repair: Grand Prix Problems, powertrain control module, alternator output

Question
I have a 1996 Pontiac Grand Prix, 3.1L V6 with automatic transmission.  Several weeks ago, after the check engine light had been on for a couple of days, the car refused to start after being parked for fifteen minutes.  It was as if the battery was dead.  The car was jump started, and it made about four miles before laying down.  I had the car towed home.  I changed the alternator.  No help.  I tried to charge the battery.  No luck.  Battery would not charge.  I changed the battery for a new one.  The car ran fine.  About two days later the check engine light came on again.  Three days later the car refused to start - no power whatsoever.  The battery would not charge.  I took the battery back to where I purchased it, and they said, "This is a bad battery.  We have tested it.  Here is a new one."  Installed the new battery.  Car ran fine.  Two hours later the check engine light came on again.  Any ideas?  Is there anything that the electrical system could be doing to damage a battery?

Answer
Hi
If possible, did you have the code scanned to see what it was?
I would then check the charging system. The ALT SENSE fuse in underhood fuse box, 10a make sure it's good.
Make sure the Alternator output is 12v or more checked on back of alt. while running.
If so, check voltage at battery. should be fairly close, within 0.7v or lower, to allow for voltage drop.
If it IS not close, Alternator charges the battery THROUGH the STARTER. It also passes through a Rust Colored fusible links down close to the battery. Check it. Check the battery cable rom Battery to Starter.
If all this is too much for you, which it can be even for me:-), find a GOOD Auto Electrical shop in your area or a Starter/Alternator rebuilding facility. There is usually a good electrical person there who'll be able to check all that fairly quickly.
It COULD be the PCM (Powertrain Control Module aka computer) which regulates the L terminal on/off. I have also seen the wrong Alternator causing sismiar problems, as the regulator is wrong inside the alternator, and the L terminal will not allow PCM inputs.
I Hope this helps.
Just advice from experience.
Good Luck
Pawl