Validation of electric vehicle battery capacity measurements with on-board charger

Battery degradation is a main concern for electric vehicle (EV) users and a reliable capacity estimation is of major importance. Every EV battery management system (BMS) provides a variety of information, including measured current and voltage, and estimated capacity of the battery. However, the estimations are subject to the car manufacturer methodology and the measurement accuracy is unknown. This article uses extensive measurements from six diverse EVs to compare and assess capacity estimation with three different methods: (1) reading the capacity estimation from the BMS through the central area network (CAN) bus, (2) using an empirical method with external current measurements and (3) using the same empirical method with measurements coming from the BMS. The empirical method relies on a full discharge and charge cycle and measuring the total current flowing into the battery. We show that the use of BMS current measurements provides consistent capacity estimation (a difference of approximately one percent) and can circumvent the need for costly experimental equipment and DC chargers. This data can simplify the empirical method by only using an on-board diagnostics port (OBDII) reader and an AC charger, as the car measures the current directly at the battery terminals. We compare the capacity based on the current measurements with the internal BMS estimation of the capacity, and provide insight on how that can be interpreted.