Does the Volt's heater work to keep the batteries warm 24/7, even when the car's not being driven?
As long as the Volt is plugged in, the heater will work to keep the batteries warm.
If it’s not plugged in, and the batteries get too cold, when started it will run the engine to provide power that the cold batteries cannot. At the same time, it turns on the battery heater and when the batteries are warm enough, it will run off the batteries.
If the batteries get too too cold, it’s going to need to be plugged in so the batteries can warm up before it will even start. Places where that can happen likely already have outlets for plugging in block heaters...
It will also run the engine to provide heat for the cabin below a selectable 35F or 17F. In this case it runs the engine until the coolant temp reaches 150F, then turns off the engine until the coolant temp drops to 120F, at which point it runs the engine again. It does have an electric heater but it uses quite a bit of power so it’s probably better to use the engine for heat once it gets cold enough.