I have not be able to figure out if there is a DEF fluid heater that runs when the truck is off. DEF can freeze so what happens overnight when it's 10F outside? I know the manual it says not to add DEF fluid in the winter and then shut-off the truck. The DEF fluid pump at the truck stop has a quilted blanket all around the pump with a flap you can open to remove the nozzle. So people obviously work about it freezing. I would think the vehicle battery would discharge in a few days in the winter if battery power was used to run the DEF fluid heater when the truck is off.