Electrochemical impedance spectroscopy for onboard application: Impact of the load conditions on the impedance relaxation behavior
Yilei Lin, Cristina Grosu, Markus Schreiber, Manuel Ank, Markus Lienkamp
Abstract
Accurate battery state estimation is critical for the safe and efficient operation of electric vehicles. Electrochemical impedance spectroscopy (EIS) has shown strong potential for this purpose, offering detailed insights into battery behavior. However, for EIS measurements to accurately reflect the true battery state, a sufficient relaxation period is essential, as impedance evolves over time after discharge until a stable state is reached — posing a significant challenge for onboard applications. Additionally, the impact of prior load conditions on this relaxation behavior remains largely unexamined in current research. This study systematically investigates the impact of depth of discharge (DoD), state of charge (SoC), and discharge profiles on the relaxation of the impedance. EIS measurements were conducted after different relaxation intervals, up to 46 h post-discharge, and analyzed at two characteristic frequencies associated with charge transfer and diffusion processes. The influence of DoD and SoC on impedance relaxation behavior has been systematically decoupled in this study. The results show that higher DoD leads to greater impedance changes during relaxation, whereas the smallest impedance variation occurs at 50% SoC. Furthermore, the starting SoC before discharge also influences relaxation behavior, likely due to phase transitions and intra-particle diffusion dynamics. The type of discharge profile also significantly affects relaxation behavior. Test cases using dynamic load profiles from real-world driving scenarios reveal that the final discharge phase before the end of discharge plays a crucial role in relaxation dynamics.