I wouldn't use the ADC 9 inch motor at such low voltage because the motor makes a lot of torque per amp, which means not a lot of rpm per volt. I would consider the minimum voltage to put that motor to good use to be 120 volts. Perhaps as low as 96 volts would be acceptable if the vehicle, complete with batteries, weighed under 2000 pounds.
With a 78 volt system I think you will find that the motor is always running warm at a low rpm. It will be well outside of its most efficient range.
With a 78 volt system I think you will find that the motor is always running warm at a low rpm. It will be well outside of its most efficient range.