How do You Group 18650 Cells into the Same Grade?

When packaging lithium battery packs (for power tools, power storage systems, and any other EV stuff), grouping together 18650 cells into the same grade is one of the most important steps. Cells from the same manufacturer in the same batch can still have significant differences that will follow through: capacity, internal resistance, voltage sag, etc. That’s why if cells are not grouped together properly, the battery pack is going to see that reflect in terms of imbalance, cycle life and even safety.
So how do real manufacturers group together 18650 cells into the same grade? Here’s one common method.
1. Pre-screening
Visual inspection is a requisite before testing anything electronically: torn sleeves, crushed cans, dirt, corrosion, failure to match printing labels, are all grounds for removing them from the run entirely. Cells that pass this initial inspection can now be cleaned and prepared for testing.
Here is where many packers will take down information like model number, batch code, supplier and that sort of information, which will later help organize and keep the run “traceable” during the cell run sort.Next, we separate the overcharged cells from the rest. This is usually done with a battery sorting machine (or semi automatic battery sorting machine), depending on scale of the production. The cells are often sorted into narrow bins with voltage ranges of +/-0.01V or +/-0.02V
3. Capacity testing in a controlled environment
Of all the testing and grading parameters, capacity is the most important for proper and accurate grading of the 18650 cell. The test doesn’t require too much in terms of sophistication, charging and then discharging each cell at a standard current, voltage, and temperature, to accurately determine its real usable capacity in mAh (or Ah).
In a properly run factory with scale, this step will be done with a battery capacity sorting and testing equipment along with lithium battery sorting machine. The tester logs its data automatically and attaches it to the row of cells as a separate file.
After this step the cells will be grouped typically in grades of:
2450-2470mAh
2470-2490mAh
2490-2510mAh
Use of closer capacity range designations produce a stronger quality and consistency in the final set or pack.
4. Internal Resistance IR measurement
This is another parameter that can affect the heat generation and capability to supply current of the cells. Even in cases where two cells would end up having a near identical capacity, a wide differential in IR could produce imbalance during the charge and discharge cycles.
IR testers like those that accompany a battery sorting machine are often based on AC or DC IR testing methods, most employing pogo pin probes for stable contact. A good IR tester for battery sorting machine will produce fast, repeatable results.
Grading is logical and simple to do. In practice the logic is usually something like:
Same capacity range
IR difference in limits for this or that (e.g. less than or equal to 2-3 mΩ)
Cells that fail will be downgraded and/or rejected.
5. Multi-Parameter Grouping and Grading
Not all battery cell sorting across the planet are of the ultra-professional and specific variety. Apart from raw productivity gains, many stick to doing it the old school way, still doing it manually (to whatever extent the cells can fit in or be accommodated in the system) or with semi-automatic “manual” cell sorting machines tied to the process display board. For custom, specially made low volume batches, manual sorting or even less automated semi-automatic machines for sorting cells is acceptable.
Professional sorting of battery cells does not rely on just one parameter. More often than not, cells are sorted according to a combination of:
The voltage
The capacity
The internal resistance
Sometimes even temperature increase during testing, or self-discharge values of the cells.
An automatic cell sorting machine, or even a lithium cell sorting machine of lower automation will be capable of this kind of multi-parameter logic, for good results.
Results of manual and automatic intelligent cell sorting logic do however, deliver a result that is indeed acceptable and consistent enough for pack assembly to go ahead.
6. The Reason behind Proper Grading
There are many benefits to grouping a cell into same germ group:
a better balanced and stable pack
longer battery life on cycles
less chance of overheating
greater customer satisfaction with less after sales service hassle!
It might not be a crucial step behind producing small run vape battery packs, but for solar battery pack and grid storage battery systems it is hard to imagine that grade sorting had insufficient pressure on it.
7. Picking the sorting machine
How well one grades the cell depends a great deal on the equipment used. Dealing with a reliable battery sorting machine means that one has channels, most importantly for stable, accurate testing of a capacity, and also reliable for proper internal resistance testing. A proper piece of equipment will:
be flexible enough for the grading logic not to blow up the sort table.
log the data for tracking quality, real good. Null point calibrate for.
Finally we issue and cheer for the cell grouping efforts which netted the great result of:Grouping all the cells in one row evenly into the same germ group. But, we sort of kid. Seriously. The job of grouping the 18650 cells is at once complex and simple. An overview shows that an operator will not check only the voltage of a battery cell since by grouping a set of them it is taking the first steps at achieving its overall goal: ensuring stable and well behaved battery packs.