As the electric bike, e-scooter, and light EV markets grow rapidly, B2B buyers often face the same question:
Why do some 18650 or 21700 batteries remain stable after two years, while others swell or fail in less than six months?

The answer lies in cell grading and quality control.

1. Cell Grading Determines Lifespan and Stability

Every battery cell is tested during production for:

• Capacity (Ah) – both nominal and actual

• Internal resistance (mΩ) – affects discharge stability and heat generation

• Cycle life – number of charge/discharge cycles

• Self-discharge / leakage rate

Cells are graded as follows:

Without clear information on cell grade, B2B buyers may experience dramatic lifespan differences even with the same nominal capacity.

2. Internal Resistance Is the Main Cause of Swelling

Battery swelling is caused by gas formation inside the cell, often triggered by:

1. Inconsistent internal resistance

   • High-resistance cells heat up faster → gas formation → swelling

2. Overcharge, over-discharge, or high-rate discharge

   • Exceeding design specs accelerates degradation

3. Poor temperature management

   • High temperatures accelerate electrolyte breakdown → gas generation

For B2B buyers, selecting low-resistance, consistent cells is key to reducing after-sales issues.

3. How PACK Assembly Interacts with Cell Grading

Even premium cells can fail early if the battery assembly is poor. Common issues include:

• Mismatched cells – mixing different capacities or resistances

• Welding or spot-welding damage – uneven heat or pressure harms cells locally

• Mismatched BMS – high-rate discharge overloads cells

Top manufacturers use cell grading, resistance sorting, and aging tests to ensure all cells are matched, maximizing battery lifespan.

4. Procurement Tips for B2B Buyers

1. Choose reputable cell brands

   • Samsung, LG, Panasonic, EVE, Lishen

2. Request grading and test reports

   • Internal resistance distribution, capacity matching, cycle life data

3. Check PACK design and BMS configuration

   • Supports high-rate discharge

   • Includes temperature protection and balancing

4. Request sample aging tests

   • Monitor discharge cycles, temperature rise, capacity retention

5. Confirm after-sales policies

   • Low failure rate, clear warranty, traceable batches

Following these steps helps B2B buyers minimize procurement risk and avoid costly early failures.

5. Conclusion

Even with the same 18650 or 21700 cells, lifespan and stability vary widely due to:

1. Cell grading and internal resistance consistency

2. PACK assembly and BMS compatibility

3. Operating environment and thermal management

For B2B buyers, understanding cell grading and battery assembly processes is essential to ensure high-quality bulk shipments, low after-sales cost, and strong brand reputation.