Reading an impedance spectrum
An impedance sweep measures how a cell responds to a small signal across a range of frequencies. At high frequency the response is dominated by ohmic resistance in electrolyte, current collectors and connections. At middle frequencies one or more semicircles appear, linked to the SEI layer and charge-transfer reactions. At low frequency a sloping tail reflects diffusion of lithium in the electrodes.
Fitting an equivalent circuit model to the spectrum turns those features into numbers that can be compared against reference data for the same chemistry and format. Changes in each parameter point to specific degradation modes.
| Feature | Frequency region | Associated mechanism |
|---|---|---|
| High-frequency intercept | High | Ohmic and contact resistance |
| First semicircle | Mid to high | SEI layer growth |
| Second semicircle | Mid | Charge-transfer resistance |
| Low-frequency tail | Low | Diffusion limits, possible plating effects |
Using EIS in grading decisions
EIS results feed directly into the grade. A module with acceptable capacity but a sharp rise in charge-transfer resistance may be close to its knee point and is routed to shorter-warranty or lower-duty applications. Signs consistent with lithium plating or internal damage move a module to refining regardless of capacity.
Impedance data is also used to match modules within a string. Modules with similar impedance share current evenly and age together, which is a key reason matched string sets outperform randomly assembled second-life systems.
- Flags modules approaching the knee point
- Identifies safety-relevant degradation signatures
- Supports impedance matching within strings
- Adds a second check alongside capacity testing
Field note