These units have a zapi controller from what it looks like so:
there is no adjustable settings for voltage, it is already loaded into the configuration file in the firmware of the controller.
If you have a PC hookup you can get the factory config file and reload it into the controller and see if that fixes it.
otherwise the only thing you can change with the handset is drive parameters, or use the 'tester' function to monitor other operations for inputs and outputs. (you could go into that and see what the actual voltage is registering as and also check the percentage of the batterys). To see if they are registering correctly.