Battery Pack State of Charge Estimation Toward Transportation Electrification: Challenges and Opportunities

The state of charge (SOC) is a critical battery status parameter in a battery management system. Lithium-ion battery packs, which serve as the power source for electric vehicles (EVs), are composed of hundreds of cells that are connected in parallel and series. Therefore, it is crucial to expand the SOC estimation from individual battery cells to the pack level. However, there is limited research that examines the pack-level methods for estimating battery pack SOC, particularly in terms of incorporating an analysis that accounts for the challenges in accurate estimation. This article examines the factors that lead to inconsistency in battery cell performance, including both internal and external features of a battery pack. The existing methods for estimating individual cell SOC have been extensively examined to serve as a foundation for accurately assessing the battery pack capacity state. The principles, advantages, and disadvantages of the existing SOC estimation methods for battery packs are discussed in detail. This review intends to offer academics and engineers involved in battery technology a comprehensive understanding of SOC estimation methods at the battery pack level.