Temperature Compensation in Ultrasonic Monitoring of Lithium-Ion Batteries for Accurate State of Charge and Ageing Assessment

Traditional direct status measurement methods for assessing lithium-ion batteries, such as Coulomb counting and open-circuit voltage (OCV), provide limited information into internal mechanical changes in operando. Ultrasound nondestructive evaluation (NDE) offers a complementary approach by probing the mechanical response of cells. This study investigates ultrasound monitoring of state of charge (SoC) and degradation in lithium-polymer (LiPo) pouch cells under controlled cycling and temperature variations. Time of flight (ToF) and signal amplitude (SA) were extracted from through-transmission waveforms via cross-correlation and electrochemical data during CCCV protocols. ToF showed closed-loop behaviour and stronger correlations with SoC than SA. Repeated 0.25C cycling induced capacity fade and ultrasonic shifts, though successive temperature calibrations partially masked these changes. Tests at 15°C, 25°C, and 35°C revealed ToF shifts of up to ∼0.7 µs relative to the 25°C baseline. Therefore, a temperature correction factor normalised ToF to this reference, unifying the ToF–SoC trend at 95% state of health (SoH) and clarifying degradation signatures. These results show the importance of accounting for temperature effects in the ultrasonic signatures in support of accurately monitoring degradation.