docs: def battery_voltage(self):
          """ Returns voltage in mV """
          msb = self.bus.read_byte_data(AXP209_ADDRESS, BATTERY_VOLTAGE_MSB_REG)
          lsb = self.bus.read_byte_data(AXP209_ADDRESS, BATTERY_VOLTAGE_LSB_REG)
          voltage_bin = msb << 4 | lsb & 0x0f
          return voltage_bin * 1.1