1987 Volvo 240 Drain on battery when car is NOT on

One of my alternator belts shredded and needed to be replaced, with only one belt I was told I was fine to drive the car, however, after driving about 15 minutes at night I lost my interior lights as well as my headlights. Jumped the car to get it home. Then both belts AND the alternator was replaced and thought the car should be fine, again, after about 15 minutes the inside and outside lights are gone, even with the battery fully charged. Had the battery checked, that is not it. When the car is OFF the "battery indicator" light is on, however, when I start the car the only warning light that comes on anymore is the oil light. What is draining my battery when my car is OFF?