You don't have to leave it plugged in unless the temperature is falling to -20C/-4F.Even when my ED is fully charged should I leave it plugged in during the winter? I am in Northern Indiana and it gets pretty. Right now it's 19 degrees F. Thanks.
You should also verify that you had the 2016 service campaign No. 2016060002 done on the car. This is a software update to the BMS. In this forum, we have had ED's that have had their 12V batteries go flat even though the car was plugged in.
An electric car is not like a cellphone. Once charging is completed, the car turns the charging station power off unless the HV battery SOC falls to 95 percent or so then it would turn charging back on. But if the 12V battery goes dead - and they have - this prevents the car from ever turning the charging station power on. It also for some unexplained reason, caused the HV battery to totally discharge - totalling the car if it is out of warranty. The software update might, or might not, change things so the car periodically charges of the 12V battery from the HV battery to prevent this.
Or more simply, plugging the ED in for charging does not guarantee that it stays charged! Yeah, I know, clear as mud.