Analysis of electro-chemical impedance and state of health based on permanent expansion for prismatic batteries

The estimation of State of Health (SOH) constitutes the fundamental basis for performance optimization and fault diagnosis of battery management systems under realistic operating conditions. This study aims to investigate the intrinsic correlations among battery capacity degradation, permanent expansion rate, and Electrochemical Impedance Spectroscopy (EIS) through detailed measurement and analysis of prismatic electric bicycle batteries before and after service. Firstly, by analyzing the internal inconsistencies within the module, it is identified that there is a close relationship between SOH and permanent thickness expansion rate. Secondly, Equivalent Circuit Models (ECMs) are fitted and Distribution of Relaxation Times (DRT) analysis is conducted with varying regularization parameters to examine the variations in EIS in relation to SOH and expansion rate. Subsequently, a novel empirical model is proposed for SOH estimation by introducing the permanent thickness expansion rate as new dimension, departing from the traditional models that utilize aging cycle counts. To extend the model's applicability, data from both pouch and prismatic batteries are evaluated, resulting in high R2 values of 0.994 and 0.945, respectively. This research is helpful in comprehending battery aging mechanisms and forecasting battery lifetime, offering valuable theoretical and practical insights for the advancement of battery state estimation.