Advanced Electrical-Mechanical Hybrid Approach for Enhanced State of Charge Estimation of Lithium-Ion Batteries

State of charge (SOC) is a critical parameter for the efficient and safe operation of lithium-ion batteries (LiBs). Currently, accurate SOC estimation heavily relies on battery open-circuit voltage (OCV) characteristics and its steepness relative to SOC. However, challenges arise when the electrical model is inaccurate or the OCV-SOC curve exhibits a flat region. To address these issues, mechanical properties have been widely integrated with traditional electrical characteristics to enhance SOC estimation accuracy. Nevertheless, the shape of the expansion force-SOC curve directly influences the estimation accuracy. This study provides a detailed analysis of the adverse effect of the plateau region and nonmonotonicity in the force-SOC curve on SOC estimation, and proposes a novel electrical-mechanical hybrid SOC estimation method. The online identified OCV is employed to determine the relative positions of the true SOC and estimated SOC, thereby guiding the direction of SOC feedback correction and the adjustment of the weighting coefficients for the electrical and mechanical estimators to achieve more accurate and robust SOC estimation. Experimental results of ternary and lithium iron phosphate batteries under dynamic operating conditions show that the adverse effects of the plateau region and nonmonotonicity in the force-SOC curve are effectively eliminated, and the maximum absolute error, root mean square error and mean absolute error are all below 2%.