Diagnostic apparatus and diagnostic method

A diagnostic apparatus comprising:

• a capacitor capable of being connected in parallel with each first battery among a plurality of first batteries connected in series;
• a plurality of first switches configured to switch a connection state between the plurality of first batteries and the capacitor;
• a detection circuit configured to detect a potential difference between terminals of the capacitor or to detect a discharge current from the capacitor;
• a second switch configured to switch a connection state between the capacitor and the detection circuit;
• a changeover switch configured to switch a connection state between the capacitor and a second battery that differs from the first batteries;
• a controller configured to control the plurality of first switches, the second switch, and the changeover switch;
• and
• a diagnostic unit configured to diagnose at least one of the capacitor, a lowermost first switch among the plurality of first switches, and the second switch, the lowermost first switch being connected to ground;
• wherein the detection circuit detects the potential difference or the discharge current after the controller turns on the changeover switch to apply a voltage to the capacitor from the second battery;
• and
• wherein the diagnostic unit diagnoses at least one of the capacitor, the lowermost first switch, and the second switch.