I don't keep it plugged in. I only plug it in when the battery falls below 50%. In any event that wouldn't help charge the 12v battery because, as I understand it, the 12v battery is only charged when the car is being driven and it is a trickle charge relay so you have to drive for a while to...
I can relate to this. My 2018 i3 is now on its 4th 12v battery. I only have about 20,000 kms on it and the dealer says that is the reason the batteries are not lasting.
I have a 2018 i3 and I am now on my 4th 12v battery. It seems to fail after about 12-18 months. The dealer says the reason for this is that I am not driving the car enough! It has about 20,000 kms on it so I am only averaging about 4,000 km per year. I think that I am going to adhere to that...