Multiple time scale state-of-charge and capacity-based equalisation strategy for lithium-ion battery pack with passive equaliser

The cycle life of a lithium-ion battery pack is much shorter than that of a single cell because of their different external operating environments and internal characteristic parameters. Equalisation management is essential for reducing the difference between battery cells, improving capacity, and prolonging the cycle life. Passive equalisation is also widely used in electric vehicles and energy storage systems because of its comparative advantages including the low cost, simple structure, and easy control. Compared with the mature development of the passive equalisation hardware circuits, the evolution of high-performance equalisation strategies is relatively backward. Along these lines, a multiple time scale state-of-charge (SOC) and capacity-based equalisation strategy for lithium-ion battery packs with passive equalisers was proposed in this work. Firstly, the minimum-capacity differential model (MCDM) consisting of a cell minimum-capacity model (CMCM) and a cell differential model (CDM) was established to improve the computational efficiency and accuracy of the dynamic behaviour of the minimum-capacity cell, as well as all the other cells in a battery pack. Secondly, the SOC and capacity of a battery pack were estimated on multiple time scales with the use of a dual extended Kalman filter and MCDM. The fast time scale based on CMCM and the slow time scale based on CDM can balance the efficiency and accuracy of the estimation algorithm. Based on the accurate estimation of both SOC and capacity, a SOC-and-capacity-based equalisation strategy was designed. Finally, a high-fidelity multicell model was used to simulate and verify the proposed equalisation strategy under the guidance of actual battery pack degradation data. The extracted experimental results show that the proposed method can efficiently and accurately estimate the SOC and capacity of new and aged batteries while a high-accuracy estimation in the entire life cycle is achieved. Compared with the traditional voltage-based and SOC-based strategies, the proposed SOC-and-capacity-based strategy can reconcile the maximization of battery pack capacity, the minimization of equalisation energy consumption, and the-equalisation duration to achieve high-efficiency balancing in the entire life cycle.