Methods to determine battery cell voltage relaxation time based on cell usage history and temperature

A method of operating an electric vehicle powered by a battery system using an estimated battery state of charge (SOC), the method comprising the steps of:

• providing a look up table (LUT) for battery relax time based on offline testing of a particular battery type at different temperatures and battery usage according to the steps of soaking a battery to a target temperature, applying a charge/discharge current to the battery, and recording a relax time to the LUT for a certain temperature and current when the battery relaxes to within a predetermined amount of a steady state value;
• monitoring key-on power up events and key-off power down events of the vehicle;
• when a respective key-off power down event is detected then determining an RMS current and temperature of the battery system;
• when a subsequent key-on power up event is detected then determining an elapsed time between the power down event and the power up event;
• identifying a respective relaxation time in the relax time LUT according to the detected RMS current and temperature determined at the time of the power down event;
• comparing the identified relaxation time and the determined elapsed time;
• if the elapsed time is greater than the identified relaxation time then 1) measuring an open circuit voltage (OCV) of the battery system, and 2) estimating a state of charge (SOC) of the battery system according to the measured OCV;
• and
• if the elapsed time is less than the identified relaxation time then 1) waiting for an additional time until a total elapsed time from the respective key-off power down event reaches the identified relaxation time, 2) measuring the OCV of the battery system, and 3) estimating the SOC according to the measured OCV.