Went to Autozone and brought my battery in for testing -- battery tested fine, above 12.5 volts. Clerk did not want to come out to test the alternator though, said to come back tomorrow. So I bought a voltmeter after all following Lucid's advice, an Actron Autoanalyser CP7665 (29.99 USD). I figure it will be good to have around for testing purposes anyway.

I have tested the alternator and here are the numbers :

No electrical devices on I get 13.97
With headlights on it drops to 13.87 and stays there
With blower motor on 4 (max) it drops to 13.27

Is this drop in voltage out of range or just normal?

Now when my battery went dead I drove for a few days with headlights on, blower motor on 4 and wipers on as well with radio of course. I also listened to my radio with engine off for about one hour just recently.

I have removed the little plactic covers on the battery cells on my Interstate, and I can see the water above the cells, so I think I am good there. No oxidation on the battery posts or cables.

Car now starts fine. I let it run for about 30 minutes after jump starting it. So it seems the alternator did its job and recharged it.

Any further test I can do on the alternator and figure out what needs to be fixed, if anything.

Thanks for your input.
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'89 244DL M47 160K miles