If this helps.
I bought my 09 a couple months ago with low battery. They claimed it was due to short trips from the last owner. Took it in, they charged it up over night, 3 months later only 27% left. Brought it back in to the same guy and they replaced the battery under warranty, and ran a drain test to ensure the car had no other issues. No questions asked at Mississauga BMW.
If anything, they also gave me a brand new 5 series as a loner for a couple days. =P