NOVELTY - The device (100) has an electric current arithmetic circuit (5) detecting induced voltage by supplying electricity to a shunt resistor (10) and calculating electric current of a battery (1). The shunt resistor is connected in series with the battery. A serial-parallel changeover switch is connected to ends of the shunt resistor, which is made of metal or alloy. The changeover switch switches a connection state of the resistor to a parallel connection when an electric current is large. A sensing terminal is arranged between two electric current electricity supply terminals.    USE - Power supply device for supplying electric power to a motor of a vehicle (claimed) such as hybrid type electric vehicle i.e. plug-in type hybrid electric vehicle, hybrid car i.e. plug-in type hybrid car, and electric vehicle. Can also be used as a backup power supply apparatus for use in a rack of a computer server for a wireless base station, a mobile telephone and a home, and an electrical storage power supply apparatus for a factory and a street light power supply apparatus.    ADVANTAGE - The device reduces consumption of electricity in a simple and inexpensive manner by connecting the shunt resistor in series with the battery, so that rating of the electric current can be easily improved in an effective manner. The device ensures that the shunt resistor can be easily changed into the parallel connection, so that gain of the shunt resistor can be improved. The device improves detection accuracy and coexistence of suppression of emitted-heat amount by switching the shunt resistor according to electric current amount. The device reduces influence of thermo EMF in a vehicle. The device can detect capacity of the battery in an accurate manner, so that deterioration of the battery can be prevented, thus prolonging lifetime of the battery in an effective manner.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of a power supply device.'(Drawing includes non-English language text)'    Battery (1)    Control circuit (4)    Electric current arithmetic circuit (5)    Pre-charge relay (6)    Output terminal (9)    Shunt resistor (10)    Power supply device (100)