Equalization cell determination device and method

For lithium-ion batteries that are composed of multiple battery cells and have a flat region in the SOC-OCV characteristic curve where the rate of change in open circuit voltage with respect to state of charge is less than or equal to a predetermined value, an equalization method is used to determine which battery cells should be subjected to equalization processing. The equalization cell determination device includes an acquisition unit that acquires the voltages of a plurality of battery cells immediately after charging of a lithium-ion battery is stopped, and a voltage difference between the voltages of the plurality of battery cells acquired by the acquisition unit. The battery includes a calculation unit and a determination unit that determines a battery cell to be subjected to equalization processing based on the plurality of voltage differences calculated by the calculation unit.