A user system is provided which includes a storage device and an auxiliary power device configured to supply a power to the storage device, wherein the auxiliary power device includes a first one direction device configured to supply a supply voltage from an external power supply to the storage device, a charging unit configured to be charged by the external power supply, a second one direction device configured to selectively supply an output voltage of the charging unit to the storage device, a voltage detector configured to detect a level of the output voltage of the charging unit and to output a first control signal to the storage device, and a switching unit connected between the charging unit and the second one direction device and configured to operate in response to a second control signal from the storage device.