Multivariable State Feedback Control as a Foundation for Lithium-Ion Battery Pack Charge and Capacity Balancing

This article presents a framework for examining the impact of different balancing strategies on cell-to-cell variations in state of charge (SOC), maximum capacity, and resistance within a lithium-ion battery pack. The literature motivates this work by showing that imbalance in both SOC and maximum capacity places limitations on pack-level charge and discharge capabilities. The use of equalization circuits to mitigate differences in SOC and voltage between cells in a battery pack is well-established. The goal of this article, in contrast, is to make the case for the use of state feedback control as a tool for balancing both SOC and maximum charge capacity. Specifically, the article draws on the electrochemical modeling and control theory literature to examine two balancing strategies: voltage balancing and a proposed novel multivariable SOC/ capacity balancing strategy. The latter strategy navigates the implicit short-term trade-offs between SOC and capacity balancing by inducing a temporary SOC imbalance between cells to increase the dissipation rate of capacity imbalance. Results indicate that voltage balancing fails to eliminate capacity and resistance imbalance between cells. In contrast, the proposed method is able to eliminate charge, capacity and resistance imbalance within the lifespan of the pack, and extends pack lifespan by 9.2%. (C) 2016 The Electrochemical Society.