Battery/Alternator, power drain?

I bought a car at a really great price, and during the inquiry I was told the only thing that was wrong with it was it needed a new alternator. So, I bought one and had it put on the car, but to my surprise it did not solve the problem. My husband isn't a professional mechanic, but he does know his way around a vehicle, but he can't figure out why the battery won't keep charge. The car will run perfectly until the battery is out of charge then it shuts off and the battery is drained. He put a new alternator on it but still the same result.