Alternator Changed...Battery Still dies

I changed my alternator yesterday becasue my battery was being drained. I also completely charged the battery. Once the alternator was replaced, the car started fine with no problems. After I woke up this morning, I drove the car for about 30 minutes. On the way back home, the radio stopped working, the signal lights stopped working, and the headlights stopped working also. The car was still running fine. WHen I pulled into my driveway, I shut the car off and tried to restart it. The battery was completely dead. Do you guys know what might be the problem??