Thanks all. I only asked because I have seen the ‘turning off non essential DC message’. The jury is out as to whether leaving an i3 plugged in will maintain 12 volt battery charge. It was suggested to me that the best way to maintain 12 volt battery health is to turn the i3 on a couple of times a week. This will engage the DC to DC 12 volt converter which runs for about 30 minutes after the car is locked. A few people suggested that simply unlocking and locking the doors would also turn on the DC to DC converter.
I agree that the BMW has built in a failsafe in that simple plugging in an i3 with a discharged battery will, after 30 minutes or do, start a 12 volt battery charge. I am trying to avoid getting into that state.