1997 Chevy Lumina Failure. And we don't know why.

Our '97 Lumina with an automatic transmission seems to be having starting problems. We thought it was a routine battery failure, so we jumped it and it ran fine. To get the charge back up, we drove it around for a few miles and thought it was satisfactory, so we began to turn around. When the transmission kicked to neutral, the car lost all electrical power. We quickly jumped it again and drove it home, and pulling into the driveway it died again. I've checked all connections and cleaned the battery terminals, yet it will still not turn over. The door ajar warning sound still works, the radio will still run, all lights work, but all of these turn off when the key is put into the "on" position. From what I can tell, the car will only operate from power provided by the alternator, while none is getting to the battery. I have yet to find a diagram of the wiring system of this specific model to confirm my assumptions, so I am asking for your help in this situation.

Thank you!