Effect of Temperature and SOC on Frequency-Response Modeling of Li-ion Batteries

This paper investigates the choice of equivalent circuit models (ECMs) for modeling Li-ion batteries (LiBs). ECMs with distributed elements (dECMs) are the most common tools to analyze LiB impedance spectra. However, the choice of the model is ambiguous. In this work, we investigate the validity of 14 typical models to accurately represent the impedance spectra of the cells under various operating conditions such as cycling, various State-of-Charge, and temperatures. These models represent a more extensive set as such ECMs are degenerate, and several models are equivalent. The model parameters were fit to the experimental data using a complex nonlinear least squares (CNLS) approach. We used 7 Li-ion cylindrical cells with graphite/LiFePO 4 material in our studies and measured electrochemical impedance spectroscopy (EIS) under several scenarios: i) new cells, ii) after cycling, iii) resting at different temperatures, and iv) indented cells. The models with the worst fit were excluded from the study after each fitting set. Our study showed that two of the 14 original models are general enough to represent the tested scenarios without overfitting; therefore, these models can be used to analyze the effect of temperature, SOC, and mechanical loading.