boardco

I have a 99 audi A4 1.8T. Starting about 8 months ago, after my car sat for four months (with the battery disconnected), my car would die when it sat overnight. Once I jumped the car it would run fine and start right up the rest of the day, and if I drove late the night before it would start up the next day. It just seemed to be when it sat for an extended amount of time. I had the battery checked at autozone where I bought the battery and they said that it was fine but the alternator was overcharging the battery. I took it to a trusted shop and they said that the infinity basslink that i had installed was draining the battery, so I had it disconnected completly. This seemed to work but shortly after that it began to die again. I went back to the autozone and they said that the battery, that was under a year old, had a bad cell. I had the battery replaced, but now it is doing the same thing. I am extremely confused and need any advise.