Fetches the value of %V and returns it as volts.

def get_supply_voltage(self, dest_addr_long=None):
        """
        Fetches the value of %V and returns it as volts.
        """
        value = self._get_parameter(b"%V", dest_addr_long=dest_addr_long)
        return (hex_to_int(value) * (1200/1024.0)) / 1000