Stable grading requires common temperature, SOC, rest, and protocol, combining capacity, energy, resistance, efficiency, OCV retention, and dimensions with upstream traceability.
Make data comparable
Capacity and resistance change with temperature, SOC, rest, and contact
Before grading, fix temperature, charge/discharge rate, cutoffs, and rest. Resistance also needs common SOC, temperature, frequency or pulse protocol, and probe contact. Capacity, ACIR, or DCIR from different protocols or temperatures cannot be binned together.
Cells leaving formation or aging need thermal equilibrium. A large cell surface near room temperature does not guarantee a uniform core. Takt design should include equilibration rather than relying on algorithms to correct every thermal difference.
| Metric | Hold constant | Common error |
|---|---|---|
| Capacity/energy | Temperature, rate, cutoff, rest | Channel bias, temperature, wrong protocol |
| ACIR/DCIR | SOC, temperature, frequency/pulse, contact | Dirty probe, poor force, short rest |
| OCV decline | Initial SOC, temperature, timestamps | No thermal equilibrium, instrument bias |
Joint-metric grading
Cells with similar capacity can have very different resistance and self-discharge
Capacity-only grading can place high-resistance, abnormal-efficiency, or poor-OCV-retention cells in one module. First remove safety and quality faults, then grade by joint capacity/energy and resistance distributions, retaining efficiency, thickness, and mass as supporting metrics.
Binning limits should come from product design, module consistency, and actual distributions rather than a copied percentage. Early production should watch bimodal, long-tail, and station-cluster patterns, which often reveal upstream or channel issues better than the mean.
- —Safety screen before performance binning
- —Use joint distributions, not means alone
- —Retain original and retest lineage
Screening self-discharge
OCV drop needs timed points, thermal control, leak, and resistance context
Sodium-ion OCV change includes intrinsic self-discharge from materials and interphases and can also arise from microleak, metallic soft short, or measurement error. Standardize SOC and temperature, define a baseline and multiple later points, and retest outliers.
OCV decline with mass or leak change points to packaging. Decline with resistance and clustering by slitting tool or weld station points to metallic particles or joins. Lot-wide shift points to materials, electrolyte, formation, or temperature. One voltage-drop limit cannot serve different chemistries and times.
| Combined signal | First direction | Trace back to |
|---|---|---|
| OCV drop + mass loss/leak | Package and reseal | Seal, weld, fill port, leak tester |
| OCV drop + resistance outlier | Soft short, contact, interphase | Slitting particles, weld, wetting, channel |
| Lot-wide OCV shift | Chemistry or common process | Material, electrolyte, protocol, temperature |
Make anomalies traceable upstream
Every cell should trace to electrode rolls, assembly stations, and formation channel
Traceability should not stop at date and shift. Bind cathode/anode roll and section, separator/electrolyte lots, winding/stacking machine, weld station, oven/position, filler, seal station, formation tray/channel, and protocol ID to the cell barcode.
When capacity, resistance, or self-discharge outliers appear, cluster by station, channel, roll section, and material. If OCV faults rise for one knife position, inspect slitting burrs and particles; if the fault follows a formation channel during swaps, service equipment first.
Data integrity is itself a quality item: missing histories, mismatched barcodes, unknown protocol IDs, or retests overwriting originals should block automatic release.
Bibliography
References
- 01Voltage drop screening for defective cells during the battery formation process
- 02An Online Fault Cell Screening Method for Lithium-Ion Battery Formation Based on a Data-Driven Model with Incomplete Time-Series Data
- 03Alleviating self-discharge in sodium-ion batteries via functional dual-salt electrolytes
- 04Areal capacity balance to maximize the lifetime of layered oxide/hard carbon sodium-ion batteries
Updated: 2026-08-26