EIS Measurements for Determining the SoC and SoH of Li-Ion Batteries

In the framework of the ALIDISSI project of the Stock-E program funded by the French Research National Agency (ANR), electrochemical impedance spectroscopy (EIS) has been used as a non intrusive tool to perform in-situ diagnosis of batteries for applications dedicated to hybrid electric vehicles and grid-connected photovoltaic solar energy. EIS measurements were performed on 3-electrode coffee bag Li-ion cells during cycling with the aim of identifying the ageing mechanisms in these applications and determining impedance parameters related to the State-of-Charge (SoC) and State-of-Health (SoH) of the cells. EIS diagrams were modeled with electric equivalent circuits and, using an automatic spectra treatment, SoC and SoH prediction algorithms based on a multivariate mathematical analysis were obtained. They give a 10% SoC error on commercial LiFePO4/graphite cells and a 7% SoH error on LiFePO4/Li4Ti5O12 cells with EIS measurements performed at 20 degrees.