Same issue here when temperature went below -20C for a few days. Of course, I was using heated seats and stering, rear defrost, etc. This is normal expected use of the vehicule, right?
Nevertheless, this issue is unacceptable to me. Never had any similar issue with any other car before, so I wouldn't expect this from a brand new BMW. Specially since Efficient dynamics is recharging the battery when you are not touching the gaz pedal.
I agree with the LagunaSecaBlue summary, and would add that if the car requires me to ride it for one hour (100km) just to recharge the battery just because in fact, the battery/alternator combo is undersized, then BMW should reimburse me for the time, gaz and mileage (0,15$ / km they charge me under lease).