I don't have i-drive. However all E9x's have I believe an electrical grid heater which could be? programmable thru I-drive. No clue what its output is, but a 300W heater is not great and would pull 25 Amps. Your battery is up to 90 Amp-hr, although theoretically that should give 3+ hours of heating at 300W, I bet you get nowhere close to that.
I've got a 180 amp alternator. When cold i switch seats to max (60 seconds and it becomes too hot for me, have to turn them down), and heater is on automatic, so it will pull max. curent too.
Would be nice if seat heating would be activated upon unlocking the car, depending on outside temperature,
