alternator/battery/charging problems

Just bought a 2000 Sportage EX 2WD. Came out one morning and I had a dead battery. Jumped it off and it was ok for 2 days. Then drove it one night for about an hour (ac, lights running), came out the next morning and dead battery again. So I am thinking charging issue. The battery is 3 weeks old, checked ok at auto parts store, alternator also checked ok at parts store. A voltage meter on the battery when the car is running shows 13 V. @ the alernator it is 13.8 V. Also took voltag emeter and put neg. to the pos. of the battery and took the positive of the volt meter to the output of the alternator and it showed .80 V. Is this normal or could I have a bad wire, fuse or relay some where. Any help would be aprreciated. Thanks in advance.