The battery heater is a fixed overhead, so even at the same amperage (and twice the power), if the weather conditions are such that the heater was taking 80% of

The battery heater is a fixed overhead, so even at the same amperage (and twice the power), if the weather conditions are such that the heater was taking 80% of the available power on a 15A/110V circuit, it'll only be taking 40% of the available power on a 15A/220V circuit, meaning the charging power is 6 times greater.