What is the problem?
The battery needs heat before start and while driving, otherwise it does not charge up.
Solution: heat the battery while driving and when engine is pre heated.
Commercial solution already exists:
http://www.arcticheat.fi/arcticbatteryheater.htm
Googletranslate version into English:
http://translate.google.com/translat...er.htm&act=url
Please tell this to your BMW service and I hope someone tips München engineers that solution is already available.