A charging control method of a master device 200 receiving an operating power from an auxiliary battery device 100 or an internal battery 230, the auxiliary battery device including a battery having a first power supply and a first power supply output port for outputting the first power supply, is provided. The method includes converting a second power supply to the first power supply upon detecting the second power supply at an input port of the master device in order to provide the converted first power supply as an operating power of the master device and/or a charging power of the internal battery, and detecting a connection with the auxiliary battery device upon detecting the first power supply at the input port in order to provide the first power supply as the operating power of the master device and/or the charging power of the internal battery without voltage drop.