I've had this error many times. In fact, in both my new 335 and my X5, I've had the battery drain completely.

The X5 was the worst, as EVERYTHING is electronic, from the rear door (to access jumper cables) to the shifter (trying to put in nuetral to reposition dead car).

I've found that if I leave my Radar detector on, I always get the message in the X5. On the other hand, I can leave it on in the 335 and no problems.

My guess is the CD's are constantly spinning, as the radio doesn't seem to turn off. I've noticed that I turn off the car with one song on, and next morning, it's on completely different song. Who knows what other systems are constantly running.

ADVISE: Never use the climate feature that cools the car without you in it. It WILL RUN THE BATTERY DEAD.

Such a stupid feature - wouldn't you think BMW would shut down systems to save enough battery life to start your vehicle???