What is Acceptable IR for Lithium-Ion Cells?

Insulation Resistance (IR) is critical in the cell manufacturing and sorting processes in lithium-ion batteries, where it indicates the safety, reliability and quality of cells. IR is a measure of the resistance to current flowing through the internal components, typically between the within the cell, between the positive and negative terminals, and between the battery itself and its case. Low insulation resistance can indicate short circuits, ingress of moisture or degraded materials, and lead to potentially unsafe battery types. Knowing what constitutes a good IR value is important to battery manufacturers, sorters and quality control teams.
Why IR matters in lithium-ion cells
The rapid growing use of lithium-ion cells for applications ranging from consumer electronics to electric vehicles, energy storage systems, and of course in the industrial environment of battery sorting means that the raised cell voltages and their high energy densities. A cell with low IR may leak current internally, leading to self-discharge, overheating, or failure. Thus knowing the IR of each cell assists in identifying defective, compromised and potentially dangerous cells, helping protect those in the sorting process through to pack manufacturer. In addition, by testing IR, the manufacturer can gain a correlation of the IR with the number of charge and discharge cycles completed. Cells with low insulation resistance leak charge at a higher rate than ‘good’ cells, and thus deplete quicker.
What are acceptable IR values in lithium-ion cells?For standard cylindrical cells 18650 or 21700, IR values can be ≥100 MΩ at room temp (20-25C), while larger capacity prismatic or pouch cells can be somewhat less tolerant (50-100 MΩ) owing to their larger surface area and different constructions.
As always, IR is a temperature-sensitive parameter and rises with temperature; IR values are usually required to be room-temperature (20-25C) figures and many battery sorting machines include a temperature stabilization feature to minimize errors with IR testing. The following are all to some extent capable of having an effect on a lithium-ion cells’ IR:
Quality of manufacture: misaligned separators, contaminating electrolyte or other defects can lower the insulation resistance
Moisture and/or humidity: exposuOo9re of the cells to moisture during manufacture can penetrate cell operation and lead to formation of leakage paths, damaging them and more of a risk that corrosion arises in cells already of lower insulation resistance
Ageing of the cell: repeated charge-discharge cycles, overtemperature operation or overvoltage events can incrementally degrade the cell’s insulation resistance
Surface contamination: remaining traces of electrolyte, dust, or contamination from the handlin of the cells can lead to the pathway for leakages
The testing of IR in sorting battery stations is routine in the industrial area and requirements of the test are beginning to find their way into less rigid battery sorting machines; most of today’s new sorting machines, automatic or semi-automatic type, measure insulation resistance alongside capacity and voltage to determine the internal resistance of the cell (in quality mixing machines, a digital-to-analogue converter may be used to generate the measured current of the insulation resistance being tested). A DC voltage (usually 100 V) is applied across the terminals and between the terminal and case of the batteries and the measured leakage current then expressed in insulation resistance format in accordance with more or less ohm’s law.
More advanced machines have the cell-polar probes; for cylindrical cells, pogo-pin probes and for prismatic and pouch version cells, soft contact plates can make use of clever and sensitive tests to obtain good results. Multi-channel sorting machines allow the simultaneous testing of different cells at different stations on the machine allowing more cells to be sorted at each running of the machine. The result of the insulation resistance is then used along with capacity, to roughly categorize the cells into “passable” and “fail” so that the cell subsequently moves to the pack assembly area only if it meets safety and other companies’ requirements.
With what you know now, here is what you will test for IR to determine whether or not your lithium ion cell is working underneath:
18650 and 21700 cells: Above 100MΩ of IR at 20-25C
Larger, prismatic pouch type cells: Might be around 50 to 100MΩ at 20-25C depending on size and design; or whatever manufacturer says
Pouch type cells: Above recommended 50MΩ; or whatever manufacturer sets as the minimum IR rating
Recycled or second-life cells may be rated lower IR than standard normally, so take this into account when examining them.
If a cell does not pass the minimum rating, don’t take a risk; to consume the cells in production will likely mean other harm comes to them through trying to fit them in a cell pack, so reject it or go further and find what other defect(s) there are, so you are sure it’s no longer in your factory by the time a cell-pathing issue arises, or worse, a short occurs, or a cell catch fire.
Abiding the rules regarding insulation resistance and enjoying the safety and reliability that they encircle the battery with, “full go for it!”.
Conclusion
Insulation resistance is important and of considerable intrinsic interest. Following good practices ensures the batteries perform as needed and stay safe. If you are sorting batteries and manufacturers, you need to be concerned about SO accordingly. Testing and tagging them, and knowing what test is pregnant to fail; or “not do so well” is the key to integrating good insulation resistance cells into batches.