Engine Dies After Removing Charger

I have a 1990 Chevy full-size van that will not stay running. After leaving the lights on over night, the vehicle would not take a charge. I replaced the battery, changed the belt, and had the alternator tested. The alternator tested good. Once I replaced the battery, the car van seemed to run fine for about 20 miles. I tried to start the engine and it seemed that there was no power what so ever. I put a charger on the battery and it started immidiately. As soon as I removed the charger, the engine died again. There was no lights or nothing. What could my problem be and how do I test it? Thanks

hrm, sounds odd.... might want to take it in to a shop if you know a good one where they can run a diagnostic on the electricals