2000 Buick LeSabre battery went dead overnight. No previous problems, battery is new. When jumped, it started Ok and charged. Strange indication on DIC, showed wrong driver key indication. Checked battery voltage and it was jumping all over with different readings. Figured alternator diodes were going. Went to local Autozone and they checked and also said alternator readings were not steady, alternator was bad. Ordered new alternator. Before replacing, checked everything next day. Battery held charge, DIC now shows proper driver key, battery shows steady voltage, not moving off reading at all.
Don't want to replace good alternator. Any hints on what else to look for. Wrong DIC indication makes me think some computer module is possibly going bad. One more thing, this was the day after hurricane Arthur dumped loads of driving rain on the car. Wet something, now dried OK ??

i would clean the battery terminals and the big wire terminal on the alt with sandpaper or a wire brush and reassemble and put dielectric grease on the connections. you get strange indications when the battery voltage is low, i wouldnt worry about the other stuff. see what happens after you clean and grease it and it rains again

Also remove and check and clean the negative cable under the rear seat where it is bolted to the floor pan.

Battery terminals are clean, battery is almost new and is under rear seat, no corrosion at all. Am thinking maybe serpentine belt might have been slipping. Has been changed at least once, maybe twice (mileage 125000). Cheap enough to replace, a lot less than the alternator.

Thanks for the hints on what to look for.

I now feel stupid. I talked to a friend who I found out is a GM trained Buick technician. He agreed with the comment about a loose negative connection. I only looked at it for corrosion, but didn't check it close enough for it being tight. It wasn't tight! He explained all the weird DIC indications were a protection GM has for when the voltage gets low, it shuts down computers controlling "non essential for running circuits". The car will continue run OK but you lose misc. displays. It also explained why I was able to jump start the car because the location for jump start is under the hood, not at the battery. So by jumping, I was putting a good positive and negative battery connection to everything and substituting it for my underseat battery. If all goes well from now on, I'll post again in few weeks to verify that the problem is really fixed.
Thanks for all the help, next time I will be more careful in following suggestions.

good to hear its fixed

Everything has worked fine since I tightened the battery cable. Strange how this simple thing showed so many weird symptoms. Thanks to all for pointing me in the right direction. Saved me the cost of a new (and not needed) alternator.

Thanks for updating us on your findings, glad it is working well now!