voltage on drum brakes

[paddler67]

I'm redoing the brakes on a trailer.
how many volts should there be at the drum brakes of a 24' travel trailer.
I had 3.2 volts at the 7 pin plug at the back of the truck regardless of how I set the brakes controller on a 05 f350 with the built in brake controller I didn't have any voltage at the trailer drum brakes. the trailer battery was dead.
do you need a good battery to have voltage at the brake drum.
thanks