docs: def get_battery_level(self):
          """Reads the battery level descriptor on the device.
  
          Returns:
              int. If successful this will be a positive value representing the current 
              battery level as a percentage. On error, -1 is returned. 
          """
  
          battery_level = self.get_characteristic_handle_from_uuid(UUID_BATTERY_LEVEL)
          if battery_level is None:
              logger.warn('Failed to find handle for battery level')
              return None
  
          level = self.dongle._read_attribute(self.conn_handle, battery_level)
          if level is None:
              return -1
          return ord(level)