A Framework for Analysis of Lithium-Ion Battery Pack Balancing Including Cell Parameter Heterogeneity

Abstract

This paper studies the impact of battery pack parameter heterogeneity on active balancing methods. Lithium-ion battery packs are often composed of multiple individual cells connected in series and parallel to meet energy storage requirements for a given application. Due to manufacturing tolerances and environmental conditions, variations in each cell's parameters are induced, which can propagate charge and other state imbalances. This is detrimental to pack operation, as cell-to-cell variations can limit total capacity and increase degradation. While studies in the literature propose balancing methods to equalize all charge levels, there are limited studies that explore analytical relationships between parameter and state heterogeneity. This paper provides a framework to model state and parameter heterogeneity simultaneously, supporting an enhanced understanding of cell mismatch effects during pack operation and balancing. The developed heterogeneity model facilitates derivation of analytical expressions relating to system stability in the presence of cell mismatch and state heterogeneity generated by parameter heterogeneity. These expressions are validated through simulation case studies of a pack represented by interconnected equivalent circuit cell models with capacity heterogeneity. For both no balancing and state of charge (SOC) balancing, results indicate that capacity heterogeneity propagates SOC imbalance while the pack is operating with a nonzero average current. Using the heterogeneity modeling framework, a modified SOC balancing strategy is proposed to equalize cells with capacity differences.