Novel Battery Model Employing Variable Capacitor for Effective Description of SOC Behavior

High-capacity batteries connected in series and/or parallel in the battery energy storage system (BESS) and electric vehicle (EV) require cell balancing circuits to ensure their lifetime and safety. Conventionally, a sequence of circuit simulations or tedious hardware tests are the only ways to evaluate the performance of the balancing algorithm. However, since the battery capacity is large, it takes much time to examine the entire balancing process under various combinations of initial voltage conditions. Moreover, it is difficult to estimate the balancing time due to the non-linear relationship between the open-circuit voltage (OCV) and the state of charge (SOC). To mitigate the problem, this paper proposes a variable capacitor model to simplify the SOC-OCV curve for rapid evaluation of the balancing time. Compared to the conventional variable voltage-source model, it can provide a rapid estimation of the balancing time with high accuracy by simple calculation of the R-C time constant based on the concept of charge-equivalent capacitance. The proposed method is useful for active balancing circuits as well as passive balancing schemes. After validating the model accuracy by experimental results, a switched-capacitor active balancing circuit and passive cell balancing chip are implemented to verify the effectiveness of the proposed model.