Equalization cell determination device and method

An equalization cell determination device that determines, with regard to a lithium-ion battery that is made up of a plurality of battery cells and that includes a flat region in a state of charge (SOC)-open circuit voltage (OCV) characteristic curve at which a rate of change in the open circuit voltage with respect to the state of charge is no greater than a predetermined value, which battery cells are to be subjected to equalization processing, the equalization cell determination device being configured as an electronic control unit comprising:

• a processor;
• and
• a memory storing instructions to cause the processor to
• acquire, via a voltage sensor disposed on each of the plurality of battery cells, each of voltage values of the plurality of battery cells immediately after charging of the lithium-ion battery is stopped and while polarization effects in the lithium-ion battery remain;
• extract a minimum voltage value among the acquired voltage values of the plurality of battery cells;
• calculate voltage value differences between the minimum voltage value and each of the acquired voltage values of the plurality of battery cells excluding the minimum voltage value;
• compare each of the voltage value differences with a predetermined threshold value;
• determine whether there exists a battery cell among the plurality of battery cells whose voltage value difference from the minimum voltage value is equal to or greater than the predetermined threshold value;
• set the battery cell among the plurality of battery cells whose voltage value difference from the minimum voltage value is equal to or greater than the predetermined threshold value as a target battery cell to be subjected to the equalization processing in a case where there exists the battery cell among the plurality of battery cells whose voltage value difference from the minimum voltage value is equal to or greater than the predetermined threshold value;
• and
• determine that the equalization processing is not necessary for all of the plurality of battery cells included in the lithium-ion battery in a case where there is no battery cell having a voltage value whose voltage value difference from the minimum voltage value is equal to or greater than the predetermined threshold value.