Electrochemical Impedance Spectroscopy for Retired Packs

Electrochemical impedance spectroscopy perturbs a cell or module with a small alternating current or voltage and measures the response across a sweep of frequencies, often from millihertz to kilohertz. At each frequency the instrument records how much the cell resists the signal and how far the response lags behind it. Plotting those results produces a spectrum, commonly shown as a Nyquist plot.

The shape of that spectrum carries information about the cell's internals. The high-frequency intercept reflects ohmic resistance from electrolyte, current collectors and connections. The semicircle in the mid-frequency range relates to charge transfer and interface layers, and the low-frequency tail reflects diffusion of lithium within the electrodes. Fitting the spectrum to an equivalent circuit model turns those features into comparable numbers.

Retired cells age in different ways. A rise in high-frequency resistance can point to weld or contact degradation, which may be fixable at module level. Growth in the mid-frequency arc usually indicates thickening of the solid electrolyte interphase or cathode surface changes, which reduces power and signals continuing degradation. Changes in the low-frequency region can suggest diffusion limitations associated with loss of active material.

This matters for second life because two modules with the same capacity can have very different futures. A module with stable impedance may age slowly in stationary service, while one with rising charge-transfer resistance may fade quickly. EIS helps separate those cases and allows modules to be matched by impedance, which keeps strings balanced in a rebuilt system.

A full capacity test can take many hours per module, while an impedance sweep takes minutes. That makes EIS useful as an early screen: modules with clearly abnormal spectra can be flagged for refining before they occupy cycling capacity, and the rest can be prioritised for full testing. Over time, data from both methods can train models that predict capacity from impedance more reliably.

EIS results are sensitive to temperature and state of charge, so measurements must be taken under controlled conditions to be comparable. A good grading line fixes both, records them with each result and repeats measurements when readings look inconsistent. The spectra are stored against each serial number so that buyers of second-life modules can see the evidence behind the grade.