Where does the PFC read the voltage from?

My battery checked out with 12v with a multimeter and was also hooked up to a running car with jumper cables, but the hand controller only shows 6v and the starter wont engage..

In general, the whole electrical system is acting weird.. It could just be a dead battery, but again, where does the pfc read voltage? I want to start there and work back to the grounds and battery.

So really weird electrical issues and low PFC voltage reading turned out to be the engine wasnt properly grounded anymore (somehow). I ran a fat gauge wire to the engine and grounded it on the chassis and everything is back to normal.