The battery of the Model 3 uses power from the garage plug to warm up the cold battery first. Then, once it has warmed up the battery (which even in typically unheated Ontario garages in the winter takes minutes not hours) the car then proceeds to actually charge the battery. In the actual charge phase, the battery IS fully charged up to its set limit, although the rate at which electrons move in the cold ambient temperature is slowed down in the winter, which is also the case when charging using Tesla's HPWC. There should be very little additional overnight time between winter and summer charging using the wall plug. I can say this from my personal experience charging my other EV (similarly designed cold battery charge protocol) in my unheated garage using just the wall plug for the past 4 winters and summers. BTW, when I plugged in my Model 3 into the garage wall, the screen showed input of 16A and 3kWh charge rate.