Investigating the Applicability of Electrochemical Impedance Spectroscopy for Parallel-Connected Lithium-ion Battery Modules

Electrochemical impedance spectroscopy (EIS) is a non-destructive method of testing that provides information on the internal processes within a cell. Previous studies using EIS have primarily focused on the cell level, with few studies conducted at the module or pack level. This study applies EIS to eight 4P1S modules at varying states-of-charge (100%, 50%, and 0%) and degrees of aging. At 100% SOC, the modules show relatively high impedance values and good distinguishability between modules with different states-of-health (SOH). As the overall SOH decreases, both the real and imaginary impedance values at the transition frequency show a clear increasing trend. Additionally, the transition frequencies cluster between 0.025 to 0.050 Hz, indicating that a small range of low-frequency signals may be sufficient for obtaining diagnostic information. For this study, a custom spring-loaded battery fixture was developed, allowing cells to be connected in parallel and series configurations without the need for welding. The repeatability of results with inserted and removed cells was also studied. The results indicate that module-level EIS tests on parallel-connected cells are strongly correlated with the overall SOH of the module and provide foundational information for implementing EIS tests at the module and pack level for state estimation, fault detection, and rapid battery sorting.