BMW Service has changed my battery four times now. But battery is not the actual problem, its the battery usage. If you have your keys inside the lock, it will start draining the battery. Eaven if you unlock the doors via remote, you can hear dvd-drive starting (along with computer and other stuff).

Battery usage is so big that when I turned ignition to first position and put the battery to charger, it consumed a lot more than the charger could charge.

ps. If you move your electric-seat too much before the car is on, it will not start .