[RoundelM3]

Did you "register" the new battery when you installed it? If you didn't, the car's charging system won't charge the battery correctly. My wife's 2011 128i had its battery go completely dead with no previous warnings right at the 3-year mark this past March, and it was replaced under warranty at our local dealership. Zero problems since, so I'd second the recommendations above and take it back to your dealership. The least they can do is get the new battery registered or replace it and you can get a refund on the one you purchased.