Influence of relaxation time on Electrochemical Impedance Spectroscopy measurements in Electric Vehicle charger

Electrochemical Impedance Spectroscopy (EIS) is a powerful tool for assessing the State of Charge (SOC) and State of Health (SOH) of lithium-ion batteries. By conducting EIS measurements following a complete charge cycle, it is possible to monitor battery condition and gain detailed insights into the cell's internal degradation mechanisms. Integrating EIS measurements into the charging process of Electric Vehicles (EVs) could enable on-board diagnostics. However, the timing of these measurements significantly affects the results, highlighting the need for an appropriate stabilization period of the relaxation time. This period, which allows transient effects within the cell to subside, may extend beyond one hour. In fast-charging applications, such as automotive systems, waiting for prolonged rest times is usually not feasible. The study of the effects of different rest time on EIS measurements is of primary importance to implement accurate and robust real-time SOH estimation algorithm directly on-board, ensuring a proper Remaining Useful Life estimation of the batteries. Thus, this work investigates the impact of relaxation times on EIS measurements, providing a comprehensive analysis of variations across the impedance spectrum. Additionally, the study examines the influence of fast-charging protocols by assessing stabilization effects at three different current rates. Finally, the role of heating effects during testing is explored to determine whether deviations are primarily due to temperature fluctuations or other internal processes.