98 Aurora Recharging Problem

I have a 98 aurora and well I have replaced the altinator and the battery. Well what happened first was that the starter would click instead of start and well i thought that the batt was dead so i jumped it and let it run then it started up just fine, the first time. After about 20 min of sitting the car wouldn't start anymore. the usual click. Next i had the batt tested and it was fine. I then I tried let the batt charge by the car again. I hooked up a smaller batt with like 600cca that was charged and the car started slow but it started. I used this for about 4 days then started the car and went to the gas station then tried to start it again and it was dead. Later that day i decided to buy a new batt and put it in and it worked great..... for about a week then the same thing happened a dead car that only clicked no start. I checked the output voltage on the new alt and well it's about 13-15v. So what do you think????????

Sounds as if there is a constant draw somewhere in the system draining the batt. Advance diagnostics can usually trace it. For what you have already spent on parts and frustration you could have had it traced. A lot of threads in this forum dedicated to electrical problems, and very few appear to have been solved. Don't be shy about taking it in, it's not as painful as some make it sound. How much are your weekends worth?