I agree with squanto. After 16min. of turning off your vehicle, your 3-series goes into a sleep mode. In this mode, the control modules (DME, Transmission control module, General Module, etc) still take a bit of milliamps from the battery. Not driving the car often or driving it for long periods won't be enough to fully charge the battery. I recommend either buying a trickle battery charger or drive the car once a week around town or better yet, to the highway.