A battery unit (130A, 130B, 130C, 130D) configured to be connected to a power supply line via an external terminal, so as to allow the battery unit to be charged and discharged via the power supply line, the battery unit comprising: a battery cell group comprising a plurality of battery cells; a control unit configured to detect an electric potential at the power supply line and an electric potential at the battery cell group, and configured to output a control signal based on a voltage difference between the power supply line and the battery cell group; and a switching unit (131, 132) that is arranged between the power supply line and the battery cell group, the switching unit configured to switch a path, according to the control signal, between a normal path and a resistance path having a resistance value that is higher than that of the normal path and connected in parallel with the normal path, so as to control a current that flows between the power supply line and the battery cell group; wherein the control unit is configured to output the control signal such that, when the battery unit (130A, 130B, 130C, 130D) is connected to the power supply line via the external terminal, the battery cell group is connected to the power supply line via the resistance path until the voltage difference reaches a predetermined voltage difference, and such that, when the voltage difference becomes equal to or smaller than the predetermined voltage difference, the battery cell group is connected to the power supply line via the normal path, wherein the resistance path comprises: a first resistance path including a first protection resistor; and a second resistance path including a second protection resistor having a higher resistance than that of the first protection resistor, wherein, when the battery unit is connected to the power supply line via the external terminal, the control unit is configured to output the control signal such that the battery cell group is connected to the power supply line via the second resistance path until the voltage difference reaches a predetermined first voltage difference, and such that the battery cell group is connected to the power supply line via the first resistance path until the voltage difference becomes equal to or smaller than a predetermined second voltage difference that is smaller than the first voltage difference. a battery cell group comprising a plurality of battery cells; a control unit configured to detect an electric potential at the power supply line and an electric potential at the battery cell group, and configured to output a control signal based on a voltage difference between the power supply line and the battery cell group; and a switching unit (131, 132) that is arranged between the power supply line and the battery cell group, the switching unit configured to switch a path, according to the control signal, between a normal path and a resistance path having a resistance value that is higher than that of the normal path and connected in parallel with the normal path, so as to control a current that flows between the power supply line and the battery cell group; wherein the control unit is configured to output the control signal such that, when the battery unit (130A, 130B, 130C, 130D) is connected to the power supply line via the external terminal, the battery cell group is connected to the power supply line via the resistance path until the voltage difference reaches a predetermined voltage difference, and such that, when the voltage difference becomes equal to or smaller than the predetermined voltage difference, the battery cell group is connected to the power supply line via the normal path, wherein the resistance path comprises: a first resistance path including a first protection resistor; and a second resistance path including a second protection resistor having a higher resistance than that of the first protection resistor, a first resistance path including a first protection resistor; and a second resistance path including a second protection resistor having a higher resistance than that of the first protection resistor, wherein, when the battery unit is connected to the power supply line via the external terminal, the control unit is configured to output the control signal such that the battery cell group is connected to the power supply line via the second resistance path until the voltage difference reaches a predetermined first voltage difference, and such that the battery cell group is connected to the power supply line via the first resistance path until the voltage difference becomes equal to or smaller than a predetermined second voltage difference that is smaller than the first voltage difference.