What is draining the power from my car?

Had symptoms of a bad alternator-while driving the lights going dim, radio going out and gas gauge failing. Had alternator tested and was told it was bad. The battery was good. Changed the alternator and drove about 14 miles during the day time and it was ok. That night drove about 7 miles and the same thing started happening as before-lights going dim, radio and gas gauge failing until the car finally died totally. Have not taken new alternator in to see if its bad. Wanted to know what could be draining the power from this car.