[RM7]

I don't know if it's the same for cars, but typically idle is pretty terrible at charging in an airplane. I'd like to think modern cars will up the RPM if the demand increases, but on airplanes you have direct control over the propeller RPM. Usually less than 1000 results in a discharge to the battery, as it takes over that number to actually charge the battery. Sitting around idling at 600rpm can wipe out your battery relatively fast. Driving the car relatively hard is usually one of the better things to charge, given the braking and higher RPMs.

With your BMW efficiency dynamics, it does not charge much of the time, I believe there is more information on this in the manual.