An open magnetic circuit coil (6) is arranged on a line connecting a battery (1) to a charger (CH1). Furthermore, a resistor (2) is connected in series with a large-capacitance electrolytic capacitor (7) for smoothing a charger output which is connected to the output of the charger (CH1). This can suppress an abrupt change in current flowing in the connection line between the battery (1) and the charger (CH1) in connection between plugs (3a, 3b).