chubhill,
why do you say a 36 v battery will give you long run time than a 48 v? As long as the kwh rating (power consumption capability) of each battery is the same the run time should be the same and it has been my experience that a 36v & 48v battery of the same battery box size & weight will have comparable kwh ratings. Plus, the cost difference (36v to 48v) in batteries & chargers is negligible -~$100 for increased performance/productivity.
The lower the voltage the higher the amp draw (or heat - an enemy of motors, electrical & solid state devises.
80 volts systems & batteries draw even less amps.