HYBRID SOC ESTIMATION AND SINGLE- INDUCTOR ACTIVE CELL BALANCING OF LITHIUM-ION BATTERY PACK

Abstract

Lithium-ion batteries have been widely used in electric vehicles, renewable energy generation systems, and portable electronics because of their energy storage capability, long term operation, and efficiency. However, there are several problems associated with the use of lithium-ion battery packs, such as SOC estimation error, cell imbalance, energy loss, and battery degradation due to an inaccurate charging/discharging process. Therefore, an effective BMS can provide improvement on battery performance and efficiency. In this dissertation, an innovative SOC estimation method and an intelligent single-inductor active cell balancing scheme for lithium-ion battery packs will be designed using MATLAB/Simulink. The combination of Coulomb counting and the Kalman filter method can result in higher accuracy in estimating SOC and minimize SOC estimation error. Moreover, an adaptive PWM balancing control scheme will also be implemented to balance the difference in SOC among cells. The active balancing control uses a single-inductor active balancing approach to transfer the energy from the cells with high SOC to those that have low SOC. Thus, it minimizes balancing energy loss compared to other passive balancing schemes. The Battery Management System model as a whole is simulated using MATLAB/Simulink software for a battery pack containing four series-connected lithium-ion cells. Different simulation results, like SOC estimation output response, voltage balancing effect, balancing current, and varying PWM duty cycle, are presented with the objective of evaluating the performance of the system. Based on the different results found through simulations, it could be inferred that a hybrid approach to estimate SOC would be more efficient. Additionally, an intelligent active balancing method reduces cell imbalance along with energy dissipation while balancing. This technique will improve the efficiency and performance of lithium-ion batteries.