[TD-Linux]

All electric cars already do this, either with resistive heaters directly on the batteries or with water heaters in the coolant loop.

[hgomersall]

Presumably once going the internal losses keep it warm?

[randomfrogs]

Depends on the car. My Bolt battery takes a LONG time to get warm just from driving, even with the battery heater running. You can speed it up a bit with yoyo driving (speed up to use power, then regen brake to dump power back into the battery, repeat as necessary) for about a half hour.