I have noticed that I can not allow the car to sit for more than 2 weeks with out being on a tender. I found that out when the car was months old, and sat unattended for 2 weeks. If the car is going to be sitting for more than a week, I plug it in. I have not experianced any other problems with the battery. I also drive less than the OP.
`08 E90 335i Sedan | Titanium Silver | Black Dakota Leather | Premium | Sport | Cold Weather | Sirius | USB/iPod | Comfort Access | Steptronic w/ flappy paddles