Bidirectional Active Equalization Control of Lithium Battery Pack Based on Energy Transfer

Abstract

Aiming at the energy inconsistency of each battery during the use of lithium-ion batteries (LIBs), a bidirectional active equalization topology of lithium battery packs based on energy transfer was constructed, and a bivariate equalization control strategy of adjacent SOC difference and voltage is proposed according to the corresponding relationship between open circuit voltage (OCV) and state of charge (SOC). The energy transfer between the inductor and the lithium battery is realized through the combination of the main circuit and the secondary circuit. Based on the Buck–Boost equalization circuit, the pulse width modulation (PWM) drive signal duty ratio is adjusted to improve the equalization speed and efficiency. The SOC is estimated by the unscented Kalman filter (UKF) algorithm, and the SOC of each single battery is estimated and sorted. The results of charge and discharge and static simulation and test of lithium battery show that the SOC difference between each cell is controlled within the threshold value of 3%, the voltage range is controlled within the range of 0.01 V, and the equalization speed is increased by 51% compared with the traditional unidirectional transfer of inductive energy balancing method. The change trend of the test results and the simulation results show a good consistency, avoiding the overcharge and overdischarge of the battery pack, and reducing the inconsistency. This method can complete the energy balance management of the battery well, the efficiency is relatively high, and the service life of the battery is improved.