Lifetime Performance Analysis of Imbalanced EV Battery Packs and Small-Signal Cell Modeling for Improved Active Balancing Control

Abstract

Conventional battery balancing techniques target fast voltage or state-of-charge convergence while improving circuit-level metrics such as power density, efficiency, and component count. System-level metrics, including prolonged pack cycle life, reduced degradation rate, and increased energy capacity, are often overlooked. In this work, the lifetime discharge energy of imbalanced battery packs is quantified and compared using Monte-Carlo-style battery lifetime transient simulations with experimentally captured drive cycles. Computations were carried out across <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&gt;$</tex-math></inline-formula> 20 000 core-hours on the Niagara supercomputer at the SciNet HPC Consortium. Imbalance was introduced by sampling the cell capacity and impedance values from normal distributions with up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sigma$</tex-math></inline-formula> =5% capacity/impedance variation at the beginning-of-life. Conventional balancing techniques are found to have little lifetime energy benefit over no balancing. A linearized equivalent circuit model (L-ECM) technique is introduced for small-signal analysis of battery packs with arbitrary capacity and impedance imbalance. An L-ECM-based balancing control is found to have a lifetime discharge energy improvement of up to 53.2% in the worst-case pack lifetime energy over no balancing. The L-ECM balancing control is demonstrated experimentally to provide 9.2% increased single-cycle discharge energy compared to no balancing in a Tesla Model S battery module.