Power supply protective device, power supply device and switch failure diagnosing method

A power supply protective device comprising:

• a first switch provided between a first terminal to which at least one of a load and a charger is connected and a second terminal to which a power supply is connected;
• a switching circuit connected in parallel to the first switch and including a second switch and a voltage drop element connected in series to the second switch, the voltage drop element causing a voltage drop of a reference voltage by a current flow;
• a voltage detection unit which detects at least one voltage of a first voltage as a voltage of the first terminal, a second voltage as a voltage of the second terminal, and a third voltage as a voltage at a point between the second switch and the voltage drop element;
• and
• a control unit which gives, to the first switch and the second switch, an open instruction to bring the switch into an open state and a close instruction to bring the switch into a closed state to control switching, and obtains a voltage from the voltage detection unit,
• wherein the control unit, (1) when obtaining an open voltage in a case where the second switch is brought into the closed state and the first switch is brought into the open state during discharging of the power supply, and obtaining a close voltage in a case where the second switch is brought into the closed state and the first switch is brought into the closed state during discharging of the power supply, diagnoses the first switch as having a failure or not on the basis of the open voltage and the close voltage,
• (2) when executing first instruction processing of giving the close instruction to the first switch and the second switch, if a voltage difference between the first voltage and the second voltage detected by the voltage detection unit during execution of the first instruction processing is less than a first threshold voltage, determines that the first switch has no open failure, and/or when the third voltage detected by the voltage detection unit during execution of the first instruction processing is not less than a second threshold voltage, determines that the second switch has no open failure.