Why Sorting and Matching Cells Is Critical When Building Battery Packs
This article explains the importance of cell sorting and matching in lithium battery pack manufacturing, including objectives, testing methods, engineering considerations, and quality control guidelines.
Category: Manufacturing & Engineering • Author: Huawen New Power
Introduction
When building a lithium battery pack, multiple individual cells are connected in series or parallel to achieve the required voltage and capacity. Small differences between single cells can significantly influence the performance of the full pack—leading to imbalance, accelerated aging, and safety hazards. Therefore, sorting and matching cells before assembly is essential for reliability and longevity.
Main Objectives of Sorting & Matching
- Ensure consistent capacity and internal resistance between cells in each parallel/series group
- Reduce voltage and temperature deviation during operation
- Extend battery cycle life and maintain stable performance
- Improve BMS efficiency and balancing capability
- Reduce failure rates and after‑sales risk
Key Parameters for Sorting and Matching
| Parameter | Meaning | Typical Tolerance |
|---|---|---|
| Capacity (mAh) | Determines usable energy | ±1% – ±3% |
| Open‑Circuit Voltage (OCV) | Indicates SOC condition and uniformity | ±5 mV – ±20 mV |
| Internal Resistance (DCIR) | Affects current sharing and heat generation | ±1% – ±5% |
| Self‑discharge / leakage | Affects long‑term stability | Below spec limit |
Typical Cell Sorting & Matching Process
- Initial inspection: Remove damaged or defective cells
- Activation / pre‑cycling (optional): Normalize cell state
- OCV measurement: Group based on voltage ranges
- Capacity testing or estimation: Discharge testing or fast measurement
- Internal resistance (DCIR) measurement: Four‑wire test or impedance analyzer
- Final pairing: Select cells with minimal tolerance differences
Recommended Testing Equipment
Battery Analyzer / Testing Cabinet
Used for charge‑discharge cycling and capacity measurement.
DCIR Tester
Fast internal resistance measurement for production sorting.
Automatic Sorting Machine
Integrates OCV, DCIR, capacity estimation, and grouping.
Temperature Chamber
Improves accuracy under controlled temperature.
Engineering Considerations
- Sorting limits must match BMS balancing capability
- Parallel cells should come from the same production batch
- Design must allow test sockets and traceability labels
- Medical / ESS / UPS applications should enforce strict standards
Consequences of Skipping Sorting
- Uneven current distribution in parallel groups → overheating
- Overcharge / over‑discharge in series groups → safety risk
- Premature aging and reduced capacity retention
- Increased chance of thermal runaway
Conclusion
Sorting and matching cells is a fundamental step in building safe and high‑performance lithium battery packs. By using proper testing procedures and matching rules, companies can improve product reliability, extend cycle life, and avoid costly failures. Huawen New Power provides engineering‑level customization and battery assembly support for industrial applications.
