Stop in for a cup of coffee

Morning!
-31° this morning. Just plugged in the block heater. This is a test. If batteries are low this morning im thinking i need an alternator. Extreme low temp causes a draw in the alternator. Can easily check it with my flir camera.

If truck is plugged in all night, the heat from the block is keeping alternator warm enough to keep it from discharging. Just a theory.
My old caravan would discharge battery when it was -10 or colder. I tried to pinpoint the draw,ant finally gave up as it was fine when warm. Swapped alternator and was all good. Ford are common too.