Returns voltage in mV

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