Abstract: The inconsistency of cell in the electric vehicles' lithium-ion power battery pack leads to decrease the battery pack's capacity and lifetime, and even seriously affect the electric vehicles' performance. An active equalization system was designed based on the bi-directional Buck-Boost topology. A novel active equalization control strategy was proposed to balance the lithium-ion battery pack in the process of charging and static state, which extreme learning machine (ELM) was used to predict the state of charge(SOC), and SOC was employed as the main balance criterion. The simulation results show that the proposed bi-directional active equalization method can realize the equalization target accurately and efficiently with less energy loss.

Key words: Lithium-Ion battery string, SOC, ELM, equalization control strategy