What Does 21700 Battery Grading Include?

As the use of lithium-ion batteries grows in energy storage systems, EV packs, power tools and solar applications, the quality consistency of individual cells is paramount. With cylindrical cells the 21700 format is a popular choice due to high energy density and good thermal performance over the earlier 18650 cells.
21700 battery grading performed prior to assembly into battery pack is essential for safe, reliable and long-lived use.
So, what does 21700 battery grading involve? Here’s a down to earth explanation of what is involved.
1. Capacity testing and grading
Capacity is the big one in 21700 battery grading.
Capacity of each cell is tested and the result compared to each of the other cells. During the tests, cells are either manually or automatically charged and discharged in more or less controlled conditions. If they vary at all, a 3500 mAh battery will not be 3500mAh when it comes out the other end; cells in the same batch commonly can vary hugely due to materials in use, how long its been since cell manufacture, aging, etc.
After the test cells are sorted and graded into ranges of capacity, ie:
4900-4950mAh
4950-5000mAh
5000-5050mAh
Cells with similar capacities will be made into the same battery pack to reduce imbalance. The result is what it is and affects
Consistency in the pack
How much energy is available
Cycle life
Professional 21700 battery sorting machines automatically test capacity and as a result
sort cells into multi channels more or less accurately.
2. Internal resistance 21700 battery grading – IR measurement
The second core grading process relates to IR. Internal resistance measuresUsually interpreted to mean:
More heat generated
Discharge efficiency reduced
Aging accelerated
For high power, high current use (power tools, EV battery packs etc) low and stable IR are necessary.
In 21700 we see that all cells are tested on:
AC internal resistance testing
DC pulse testing (some quite fancy hardware)
Then graded against IR in ranges, often combined with capacity of the cells. Some of the more modern lithium battery cell sorting machines do use pogo pin or probe contact systems to guarantee stable relatively repeatable reading.
3. Voltage Sorting (Over Circuit Voltage Sorting)
Open Circuit Voltage (OCV) sorting is almost always the very first screening step.
For 21700 cells—before any capacity tests—they are measured for:
Initial voltage
Consistency of voltage in a batch of cells
Any cells which are severely out (too high, too low) will probably be one of:
Over-discharge
Over-charge
Internal defect or aging
Early skips mean these cells will not waste time on valuable test stations, and also means extra safety margins. OCV sorting is also useful in:
Inspection of stock in warehouses
Second life battery stock evaluation
Incoming quality control
4. Self Discharge Rate Testing
Often overlooked, can be important when buying for long life.
Cells charged and must stand for a specified time (7-14 days) before measuring plant gain voltage.A high self-discharge is usually a symptom of:
Internal leakage
Micro short circuits
Poor stability of the cell.
Cells like this are generally rejected from pack assembly, especially when it comes to:
Energy storage systems (ESS)
Solar battery packs.
More advanced battery grading systems take care of this automatically alongside their other self discharge recording services.
5.Consistency Matching (Multi-parameter sorting)
21700 battery grading should not be judged on one parameter alone.
In real production the cells are matched up using more than one data point to achieve what’s called consistency matching:
Capacity
Internal resistance
Voltage
Self-discharge behavior
And professional automatic cell sorting machines match alongside these parameters in multiple data channels in order to form tight packs. Benefits of this work are:
Less work needing to be done by the BMS to balance the pack.
Improved safety of the pack.
Longer overall battery life.
In really premium grade batteries tolerances can get extremely tight.
6.Temperature Behavior (Optional but important)
Certain manufacturers and pack builders make a note of whether the battery cells heat up during charge an discharge as this could be a sign of:
Higher resistance
Manufacturing defects
And while perhaps not part of the primary system of grading, this data is forming a more integral part of the data jumble of:
EV battery production
Certain types of use where there are high rates of discharge.
7.Sorting for Different Applications
Normally after the higher grade cells are matched, in grading systems, the cells are sorted into different levels of application use, such as:
Grade A cells, matched in tight capacity and IR range together and only used for EVs or high-end battery packs.
Grade B cells, with a slightly wider tolerance, and suitable for power tools or light EVs.
Grade C cells of lesser use, used for testing or low current applications for example.
Thus making the best use and least waste of each battery cell.
8.21700 Battery Sorting Machine
Manual testing for that grade type is time consuming and error prone, especially at volume. Better to have a professional 21700 battery testing and grading machine with all the work done automatically on the following order:
Automated capacity and IR testing.
Multiple channel sorting.
Multi data recording and traceability.
Less labour cost.
Higher throughput and accuracy.
Both semi-automatic test systems and fully automatic are available and used across the market.
Conclusion
21700 battery grading and screening is a very serious systemic process, and generally is approached using one of the diagnostic tests above – capacity, internal resistance, voltage, self-discharge and consistency matching depending on eventual usage.