Effects analysis on active equalization control of lithium-ion batteries based on intelligent estimation of the state-of-charge

In this paper, the characteristics analysis results of lithium-ion battery show that the essence of the inconsistency of lithium-ion battery is State-Of-Charge (SOC) inconsistency. Therefore, the disparity of SOC can be used to describe the battery inconsistency degree and investigate the equalization control strategy of lithium-ion battery. Firstly, Extended Kalman Filter (EKF) algorithm is proposed to estimate the SOC on the basis of the Ampere-Hour integral and open circuit voltage. Then, the Simscape battery model is established to estimate battery parameters, and the HPPC experiment is employed to verify the SOC estimation precision. Moreover, based on the battery SOC inconsistency laws, the battery equalization control strategy is presented. Finally, the experimental bench is set up to validate the effectiveness of the equalization strategies. Simulation results depict that with the active equalization, the dispersion of SOC drops from 9.4% to 0.84% during charging phase, while the dispersion of SOC drops from 7.4% to 4.12% and the dispersion of voltage drops from 8.54% to 0.71% during discharging phase. The experimental results show that the maximum SOC estimation error is about 4%, so this SOC estimation method meets the accuracy requirement; the 6 batteries are fully charged at the same time with charging equalization, the voltage error is within 15 mV and the voltage variance is within 2%; discharging processes of the 6 batteries are over at the same time with discharging equalization, voltage error is below 15 mV and the voltage variance is below 2%. These results indicate that active equalization control in this paper not only can improve cell inconsistency but also can improve the energy utilization of the battery pack in the process of charging and discharging.