97 Caravan Battery Drain

I am in the process of preparing my family’s 1997 Dodge Grand Caravan (3.3-liter V6, automatic, 160k miles) to find a new home. I am thoroughly cleaning the interior from 16 years of kids’ (i.e. me and my sister) spilled juices and crumbs. That is going well, but yesterday I had a new issue.

The van had been parked for several months, for we don’t need it (I drive a hand-me-down 95 Camry V6, and Mom got a 2011 Genesis sedan at my urging). When I was given the task of preparing the car for sale, I just walked up to it and turned the key. After a few moments of cranking, just a few seconds longer than normal, the engine caught and ran normally. I left it running for about 15 minutes, and it seemed to be good.

Yesterday (2 days later), I drove it to a job interview (first one!) and back, running the AC for all of the return journey of about 6 miles. I parked it, and went to get lunch. After about 90 minutes, I returned to the vehicle, and it was 100% dead. No horn, no lights, not even a “chime” to tell me my door was open. The odometer and gear indicator LCDs didn’t even light.

I took the battery out and connected it to a charger, but the ammeter (built into the charger), after initially displaying the normal 10-amp current, would drop to zero at some point. Is it possible for the battery to catastrophically fail that quickly? Did I damage the alternator by forcing it to recharge a low battery? Is is possible that there is part of the electrical system that is not powering down properly?

Thanks!