Why Do 21700 Batteries Require IR Grading?

In the fast expanding power of battery, 21700 cells will be found to play a supporting role across a growing number of applications from electric vehicles (EVs) to energy storage systems. As demand increases for a battery of higher capacity than existing solutions or with greater efficiency, so the sorting and grading of batteries is seen as essential. One of these vital methods is IR (Internal Resistance) grading.
Why is IR grading of 21700 cells essential?
Internal resistance (IR) is a key property of all lithium-ion batteries including the 21700 cell format. The higher the resistance, the more difficult the current finds it to flow, and the more power is lost in internal heat. A battery’s efficiency, or rather the proportion of its energy that can be reliably extracted, rises and falls with IR, as does the maximum charge and energy capacity at which the battery will both function safely and last long enough to be of practical use.
Grading 21700 batteries by IR ensures that they will be put together into a multi-cell pack of only the most closely-matched IR cells, thus assisting with producing a consistent IR pack design with overall efficiency and longevity of cycle and charge, prolonging the life and yield of the battery for its purpose.
Grading “How important is accurate IR grading to battery sorting?”Virtually possible inefficiencies are lessened when cells with a similar internal resistance are bunched together. This is important in electric vehicles where even small inefficiencies mean thousands of miles lost before the first battery change!
Improved Safety
IR grading helps protect the battery pack. Cells with a greater internal resistance generally generate more heat during a discharge cycle, as well as during a charge. When you place cells of differing resistor grades in the same pack, this could lead to a risk of overheating, possibly causing a fire or even a thermal runaway. Ensure all battery cells placed within the same cell pack are of the same IR grade.
Longer Life
Sorting 21700 cells by their internal resistance grades ensures they are generally equal in their discharge and charge facilities thereby preventing any one cell from being over-stressed in any one cycle. This goes a long way to extending the battery pack life and improving the cycle times.
Cheaper Manufacturing Process
Grading and sorting cells by their internal resistance also assists manufacturers in identifying and excluding the poor 21700 cells and developing a less expensive manufacturing process. That is indeed a win-win situation!
How is IR Grading carried out?
IR Grade is the measure of voltage lost when a slight load is placed across the cell terminals for a brief time. This small voltage is measured and recorded to show the IR grade.
21700 batteries are graded for their IR grade usually during 1 of the between 10 testing stages described on testing chapter. Cells graded for their IR usually do so as part of the manufacturers duty to supply the best 21700 lithium-ion cells for battery packs. Grading takes place when cells are regularly discarded after testing, so it is a mercifully short step.
In summary, there cannot be a more important requirement of the factory other than placing the cells in charge right now.