I keep losing power in my car (1989 Mustang GT) changed the bad altenator and replaced battery....?

the car ran for a day after changing those 2 items but now the battery is drained again what else could cause this???I keep losing power in my car (1989 Mustang GT) changed the bad altenator and replaced battery....?go to autozone and have a free charging system check done, it's possible you got a bad alternator even though it's new, also check to see that something isn't draining the battery when the car is shut off.