TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred System: The secondary battery system further comprises at least one resistor through which electric current flows for charging and discharging the capacitors. At least one resistor is arranged between the capacitors and the alternating current power source. The alternating current power source has a cycle of the alternating current voltage that is less than 100 times a time constant provided by multiplying a resistance value of the at least one resistor and a capacitance value of each of the capacitors. Each of the capacitors is selected from a group that includes a ceramic capacitor, an aluminum electrolytic capacitor and a plastic film capacitor. When a difference between a highest detected voltage and a lowest detected voltage of the at least two secondary batteries connected in series is higher than 50 mV, the alternating current power source outputs the alternating current voltage. An operation time of the alternating current power source is varied in accordance with the difference between the highest detected voltage and the lowest detected voltage. The secondary battery system further comprises an inverter to which electric power is fed from the at least two secondary batteries connected in series, where when the inverter stops operating, the alternating current power source operates to produce the alternating current voltage. The secondary battery system further comprises a metal case that covers the at least two secondary batteries connected in series, where the first diodes and the second diodes are arranged at a position within 10 mm from the metal case. A secondary battery system comprises secondary batteries connected in series, each having a positive electrode and a negative electrode; first diodes, each having an anode and a cathode, the anode being connected to the negative electrode of a corresponding secondary battery; second diodes, each having an anode and a cathode, the cathode being connected to the positive electrode of the corresponding secondary battery, and the anode being connected to the cathode of a corresponding one of the first diodes; capacitors connected to junction portions, each junction portion being a connection between the cathode of a first diode of the first diodes and the anode of a second diode of the second diodes; and an alternating current power source connected to the junction portions through the capacitors, the alternating current power source selectively operating to produce an alternating current voltage; where the alternating current power source uses the secondary batteries as a power source. The secondary batteries connected in series is divided into battery blocks and battery control circuits that use a corresponding one of the battery blocks as the power source; and where the alternating current power source operates to produce the alternating current voltage using the power source as a reference potential in accordance with a control signal from each of the battery control circuits. The secondary battery system further comprises switches located between the secondary batteries and the alternating current power source. Each of the switches is provided between at least one of the capacitors and the alternating current power source. The secondary battery system further comprises an ammeter for measuring an output current from the alternating current power source. The alternating current power source is a variable voltage alternating current power source that can vary its output voltage.