How to Avoid Inconsistent Capacity When Assembling 21700 Packs

Inconsistent capacity is a common problem when building 21700 lithium battery packs. It leads to pack performance degradation, shortens cycle life, creates safety risks and dissatisfied customers. Many pack malfunctions that appear after a few months are the result of poor cell consistency at the sorting stage.
This article describes the true causes of capacity inconsistency in 21700 packs and how manufacturers can reliably avoid them through proper testing, sorting and process control.
1. Why does capacity inconsistency happen in 21700 packs
Even cells made by the same manufacturer and batch are not perfectly identical, and when making large volumes, minor differences in materials, coating thickness, electrolyte distribution and temperature during the formation lead to differences in their capacity, internal resistance and voltage behaviour. Common reasons for inconsistent capacity are:Mixing cells with different actual capacitiesNot taking rise and fall of internal resistance into accountNot performing accurate/unstable capacity testManual sorting or not enough sorting channelsPoor temperature control while testingUsing cells with different agesWhen these cells are put together into a pack, the weaker cells reach their charge or discharge limit faster, causing “out of balance” cells, overheating cells or cells that “die” quicker.
2. Capacity alone is not enough
Many assemblers only look for capacity values, ignoring all other parameters. This is a huge mistake.For 21700 battery packs, consistency relies on:
Capacity (mAh or Ah)
DC internal resistance (IR)
Open circuit voltage (OCV)
Voltage drop under load
Self-discharge behavior
Cells that have similar capacity but very different IR will not behave consistently when used in the same pack. This is why modern 21700 battery sorting machines are looking for patterns that require all three measurements be made: capacity, IR and voltage.
3. Use proper capacity testing conditions
Inaccurate capacity testing (and therefore capacity sorting) is a largely invisible cause of inconsistency.
Some of the key testing conditions that must be controlled are:
Charge and discharge current: These meter match the cell specification
Cut-off voltage: If cut can’t be reliably held, results will be false.
Temperature: Widely varying capacity with temperature.
Rest time before testing: Cells should be given at least a little time to stabilize before being tested.
A real, professional-grade lithium battery sorting machine will also be able to illegally control for stable current control and temperature compensation across many machines. Manual systems will struggle and low-end machines will not be able to make quality cells, period.
4. Sort cells into narrow capacity ranges
In order to save some time, it is tempting to just process sorta of cells into wider “bins” than that specified by the manufacturer.
Example:
±100mAh → higher risk of inconsistency in pack balance, quartered, so to speak.
±50mAh or higher → much better balance.
Again, high-channel automatic cell sorting machines with high throughput allow for finely “grading” cell pack for pack without major drop off in efficiency. For high quality 21700 packs, this cell grouping approach is definitely most recommended!
5. Internal resistance matching is a must!Best practices:
Sort by capacity and IR simultaneously
Reject cells with unusual IR even if capacity is acceptable
Use IR pogo pin tests to ensure stable contacts, and that the measurement is repeatable.
Modern 21700 cell assemblers using IR as a pogo pin importantly lower measurement variance compared to clip-based or manual probing.
6. Don’t mix cells from different manufacturers or production batches
Don’t mix
Cells from different manufacturers
Cells from different production dates
New cells with aged or recovered cells
Cells with different aging traits self-discharge and degrade at different rates, and quickly lead to pack imbalance.
7. Automation removes human error
Manual sorting machines are sufficient at small scale or for lab work, but human error is unavoidable at larger quantities-. Automatic or semi-automatic battery sorting machines provide
Stable testing conditions
Consistent sorting logic
Higher throughput
Lower long-term defect rating
For commercial pack assembly of 21700 with large volumes to be accommodated, automation is about consistency, not speed.
8. Make sure you record and trace your data
Data tracking is commonly not considered, but highly valuable.
A good battery sorting system will
Store capacity and IR data
Link data to batch numbers
Capture details so analysis can be traced if you need to retrace steps quality control
When an issue appears “in the wild”, traceable data helps to know if that came with the sorting system, in assembling, or with the use, conditions, and environments the cell assembly was exposed to.
9. Checklist for consistent 21700 packs
To maintain consistent packs of 21700 battery packs:
Use accurate testing for Capacity and IR
Control temperature
Sort cells to narrow (but sufficient) cell capacity ranges
Sort cells to internal resistance, and not capacity
Don’t mix cells from different manufacturers, even if testing similar
Don’t mix between batches
Use a solid automatic or semi-automatic sorting machine
Keep complete testing and sorting records.
Conclusion
Inconsistent capacity of 21700 pack is not due to a single reason listed here. It’s a combination of weak control at the testing and sorting stage. The right cell sorting machine equipment and best standards practices are far cheaper than returns, issues, and complaints that come up later.