A Comparative Analysis of Capacitor-based Cell Equalization Methods for Li-ion Batteries

The popularity of lithium-ion batteries for energy storage purposes has increased significantly in recent years. Lithium-ion battery packs typically contain multiple individual cells connected in series to attain the desired voltage level. Over time, due to factors such as differences in manufacturing tolerances, operating conditions, and aging, some cells may develop imbalances in their state of charge, resulting in some cells being overcharged and others being undercharged. Cell equalization helps to prevent these issues by equalizing the SOC of all cells within the battery pack. This is typically done by actively transferring charge between the cells, using energy-storing elements such as capacitors or inductors, to ensure that they are all at the same state of charge. Capacitor-based active cell equalization has been studied in this paper because they have a simpler controlling mechanism as well as its implementation costs are less. In this work, switched capacitor, double-tiered switched capacitor, and chain-structure switched capacitor-based equalization techniques have been implemented and compared using MATLAB/ Simulink modelling for a series combination of 4 lithium-ion cells having 3.7V and 2.6A ratings with initial SOC variation as 15%. The cells are considered to be balanced when the maximum SOC difference between them reaches less than 1% in order to equalize the cells identically to each other. The results show that the double-tiered equalization technique takes 56 minutes less than the switched capacitor and 37 minutes chain-structure-based techniques.