Patent ID: 8085515

Claim:
An electric power source for a motor vehicle comprising: a driving battery for driving a motor vehicle, the driving battery including a positive-side battery unit and a negative-side battery unit connected in series at a connecting point; a positive-side contactor and a negative-side contactor, the contactors being connected in series to the driving battery on the positive side and the negative side, respectively; a control circuit controlling the positive-side contactor and negative-side contactor to be switched on or off; a battery unit voltage detection circuit for detecting the voltage of the driving battery to control charging and discharging of the driving battery based on a detected voltage, the battery unit voltage detection circuit being connected with the driving battery at the connecting point, an output of the positive-side battery unit, and an output of the negative-side battery unit; and a welded state discriminator detecting a welded state of the contactors at contact points thereof, wherein the welded state discriminator comprises: a single voltage detector which is capable of detecting each output voltage of both the positive-side contactor and the negative-side contactor on a loading side thereof with respect to the connecting point for the driving battery; and a judgment circuit for discriminating the welded state of the positive-side contactor and the negative-side contactor, based on an amount of and a positive or negative property of the detected voltage which is obtained by the voltage detector, wherein the single voltage detector that is connected to the connecting point for the driving battery is directly connected to the loading side of either the positive-side contactor or the negative-side contactor, and capable of detecting each output voltage of both the positive-side contactor and the negative-side contactor, and wherein the electric power source is so constructed and arranged that while the control circuit controls both the positive-side contactor and negative-side contactor to be switched off substantially at the same time, the voltage detector in the welded state discriminator detects a voltage of at least one of the positive-side contactor and negative-side contactor on the loading side thereof and then the judgment circuit compares such detected voltage with a predetermined voltage, so that when in a state of a plus voltage where the detected voltage is larger than the predetermined voltage, the positive-side contactor is judged to be in a welded state, and when in a state of a minus voltage where the detected voltage is larger than the predetermined voltage, the negative-side contactor is judged to be in a welded state, the welded state discriminator being operable to identify which of the positive-side contractor or the negative-side contactor is in a welded state based on the detected voltage by a single detection of the output voltage of the contactor.