A Joint Analysis and Estimation Effort for Cell-to-Cell Variations in Lithium-Ion Battery Packs

This article studies parameter variations in battery packs and estimation of the imbalance propagated by such heterogeneity. Battery pack use has drastically increased in several areas, ranging from personal vehicles to utility-scale power distribution. However, manufacturing tolerances allow for slight variations between battery cells, which can cause uneven current distributions and hinder pack operation. Current work in the literature studies these parameter discrepancies by analyzing their effects or estimating the imbalances, but there are scarce efforts toward combining these tenets of addressing parameter mismatch. This article presents a modeling framework conducive to both analysis and estimation, allowing for investigation of battery dynamics due to unequal parameters, providing analytical representations of the impact of cell mismatch on state and output dynamics. Furthermore, the framework facilitates the development of an online state estimator with reduced computational cost. After parameterization of 66 lithium-ion cells, the framework is used to determine the contributions of multiple types of parameter heterogeneity on output imbalances. The proposed estimator is then validated experimentally, showing how the fewer required calculations benefit estimation runtime. The results show that this estimation scheme is capable of providing estimates within 0.6% state of charge (SOC) of a baseline estimator's error while providing over a 60% reduction in computational cost.