Battery system

A method of operating a battery system installed in a vehicle, the vehicle comprising a motor-generator and an internal combustion engine, the vehicle being configured to switch between electric travel in which the vehicle uses power from the motor-generator but not the internal combustion engine and hybrid travel in which the vehicle uses power from both the motor-generator and the internal combustion engine, the battery system comprising an electricity storage device and an electronic control unit (ECU), the electricity storage device comprising a plurality of battery units connected in series, the electricity storage device being configured to receive power from the motor-generator and to supply power to the motor-generator, the method comprising:

• calculating a vehicle state of charge (SOC) and an average SOC using the ECU, the vehicle SOC being a ratio of a remaining amount of power of the electricity storage device to a total amount of power that can be charged to the electricity storage device, and the average SOC being an average of a maximum unit SOC and a minimum unit SOC among a plurality of SOCs corresponding respectively to each battery unit of the plurality of battery units;
• switching a travel mode of the vehicle between a charge depleting (CD) mode and a charge sustaining (CS) mode using the ECU, the CD mode being a travel mode in which electric travel is performed, and the CS mode being a travel mode in which an amount of power stored in the electricity storage device is maintained within a predetermined range by switching between electric travel and hybrid travel;
• causing the vehicle to travel in the CD mode when the vehicle SOC is higher than a predetermined value, and switching from the CD mode to the CS mode and causing the vehicle to travel in the CS mode when the vehicle SOC falls below the predetermined value;
• and
• calculating the vehicle SOC at a higher value than the average SOC when the average SOC is higher than a threshold value, and calculating the vehicle SOC at a lower value than the average SOC when the average SOC is lower than the threshold value, the threshold value being set at a value lower than a center value of an upper limit value and a lower limit value of the vehicle SOC.