Aging assessment of thermally regulated LFP prismatic cells using DCIR and EIS measurements

This work presents a preliminary investigation into the aging assessment in lithium iron phosphate prismatic cells under tightly controlled thermal conditions with an ad-hoc recirculating oil-bath system ensuring uniform and stable thermal boundary conditions, limiting surface temperature fluctuations below 1 °C during cycling. The study combines online DC Internal Resistance (DCIR) monitoring with periodic Electrochemical Impedance Spectroscopy (EIS) measurements, aiming to provide consistent and temperature-decoupled State of Health (SoH) indicators. DCIR is continuously extracted from current step responses, while EIS measurements are performed periodically in the 200 kHz–100 mHz frequency range and fitted using an Equivalent Circuit Model (ECM). Preliminary results on a commercial 32 Ah LiFePO4 prismatic cell at 45 °C show a linear increase in DCIR with cycling, confirming its sensitivity to aging, while capacity fade confirms the limited aging state of the cell. EIS-based analysis highlights the evolution of ECM parameters as a function of State of Charge (SoC), with consistent impedance spectra across mid-life cycling intervals, suggesting limited structural changes in the investigated range. In the early aging stage, however, the variation of the extracted parameters remained below the sensitivity of the measurement system, limiting the ability of EIS to detect subtle SoH changes. The proposed methodology demonstrates the importance of stringent thermal control for reliable impedance-based diagnostics and supports the development of simplified SoH estimation strategies suitable for real-time battery management systems.