回复：Do you mean BMW car itself or BMW owner ?

It is fundamentally correct that the alternator has to produce more electrical power than the car consumes. With a sophisticated electrical system as a BMW has, however, the battery functions not only as an energy reservoir, but also as a HUGH capacitor to smooth out voltage spikes. Suddenly disconnecting the battery while engine is running, we are expecting a voltage surge which is very much undesirable or even dangerous to modern computer controlled electronics. Please search Google by typing “disconnect the battery when a car is running.”

Nevertheless, this technique is quite useful and efficient for older cars without using on-board computers.