What is the average impedance of new 18650 cells?

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DanJohnson

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Aug 24, 2021
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I’ve found manufacturers specs that say new cells shouldn’t exceed anything from 70-100 mΩ but that doesn’t tell me what the average impedance would be for new cells. Obviously it’s not going to be 0 but I’d just like to know what y’all expect impedance of new cells to be to give me a better idea of how degraded my used cells could be based on their IR values.
Thanks!
 

DanJohnson

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Additionally would these base impedance levels differ across chemistries of L-ION cells?
 

Oberfail

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The datasheet is pretty spot on with the average mΩ of the cells when new. Most laptop cells when new, are between 40 & 70 mΩ.
Higher current draw cells like for e-bikes or vapes are more in the 50-10mΩ area.

But that really depends on which exact model of cell you have. You cant compare X Sanyo cell with Y Samsung cell pureling by the difference of their mΩ.
 

Wolf

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but I’d just like to know what y’all expect impedance of new cells to be to give me a better idea of how degraded my used cells could be based on their IR values.
18650 cell impedance/IR is manufacturer, part number, and chemistry specific. There are basically 2 different type of chemistries. ICR and INR. There are also hybrids of both chemistries.
INR/NMC are usually your high drain capable cells with a lower IR range ≈10mΩ to ≈35mΩ.
ICR/LCO are usually your low drain capable cells with a higher IR range ≈35mΩ to ≈80mΩ
Your hybrids will fall into any of those ranges but generally are "mid range" ≈25mΩ to ≈60mΩ
The best way to determine the SOH "State of health" of a specific make - model cell is through experimentation. Taking an IR reading when harvested, charging the cell and running a capacity test. I have done that to 6000+ cells, download/view spreadsheet here, and have come up with a "cheat sheet" you can download/ view here. This by no means is an authoritative document, but it is my findings on IR values that produce acceptable capacity results by cell manufacturer and part numbers. Unfortunately there is no magic single mΩ reading that encompasses all 18650 or any Li-Ion cell format. It is learned by taking the manufacturers "spec" with a large grain of salt and adjusting the mΩ readings to your findings. You will find though that "most" ICR laptop batteries that are any good will be in the ≈<60mΩ range. The difference between a good cell (considering a cell with 80% SOH as good), a marginal cell and a bad cell can be just ±5mΩ. There are some stragglers like the LG*(*)S31865(0) that will perform well up to 80mΩ but for sake or IR consistency for a battery build I would not use them.

Wolf
 
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paddy72

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Nov 17, 2021
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And if you talk of cell impedance or inner resistance (IR or Ri) you would normally have to specify if you refer to AC IR or DC IR - that can be (and normally is) a big difference! Actually the IR depends on several parameters: the frequency of measurement and the time of the load (AC or DC method, even the AC frequency is important), the temperature of the cell and the SoC (State of Charge) of the cell. SoC seems to matter the least, whereas temperature is important and most important the frequency. For most datasheets and IR measurements you will find here, they refer to the AC-method, normally 1 kHz - as many IR-testers use 1 kHz AC to measure the IR.

The DC IR is normally significantly higher, typical 2...3 times, but is also more difficult to measure, as the cell gets drained more or less and you get a voltage drop until the reading stabilizes. This can take several seconds - or event won't stabilize at all on a weak cell or to low of a load resistor attached - so that you can't get exact and reproducible readings. Thats why most people use the AC method and typically with 1 kHz.

For the sake of meaningful and comparable data is would suggest to generally use the AC 1 KHz method at 50% SoC (3.7...3.8 V) and at room temperature of 20 deg. C. If the cell is much warmer, the IR can be lower and at very low SoC the IR can be higher.
 

Wolf

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At the risk of sounding repetitive it is assumed that IR/Impedance of a cell is tested using a 4 wire 1kHz AC tester/method as described by most manufactures spec sheets.
For sake of simplicity we just write IR and again assume that the reader knows that we are talking about AC IR. This assumption may be flawed and it was good of you to point this out.
As you also point out DC IR is typically 2 to 3 times higher than AC IR although I unfortunately do not agree that it takes several seconds for the cell to stabilize during a DC IR test more like a couple of milliseconds. If a cell is of good quality it matters not so much what the load resistor value is as long as it is not a direct short to get a relative accurate DC IR reading again using a 4 wire method. The SoC of the cell is somewhat important for a DC IR measurement and for repeatability a 4Ω resistive load can be put on the cell resulting in ≈1000mA draw which if the cell is of good quality can be repeated several times with small measurement differences resulting from the resistor heating up and chemical changes within the battery as it tries to satisfy the load. Totally understandable.
An AC IR test, on the other hand, does not load the battery but uses a high frequency AC signal (1kHz in our case) injected into the battery to give us glimpse into the cells reaction to this signal, independent of the cells voltage. So in essence the SoC for an AC IR measurement is not quite as important as with DC IR.

I agree that your AC IR and DC IR measurement should be conducted in a relative conventional environment rather than at Antarctica or the Sahara. Also since an AC IR test is so easily performed I recommend doing the test several times during your harvesting and testing procedure. Initially after collecting the cells to determine if they are worthy of testing. As what is the sense of running a full C/D/C cycle on a cell that will be a low performer or a heater along with possibly being a SD.
With over 15,000 cells that have passed through my hands I have a pretty good grasp on what a cells AC IR should be no matter what the SoC is.
Yes you are correct AC IR is a bit higher on a cell with a very low SoC. Depending on the cell manufacturer and SoH of the cell this can vary but in my experience on a good cell the AC IR drop from a low SoC is an average of -4.99mΩ when fully charged. After a >25 day rest period the AC IR rises an average of 0.16mΩ with a Vdrop average of 0.156V These numbers came from 1280 LG M26 extracted from Ninebot scooter batteries. Once the build began after sorting the best of the best cells out of the 1280 cells down to 1120, with an average age of the cells being ≈200 days since the C/D/C, testing the AC IR and voltage for the final time produced these results. Now remember these cells were sorted by certain criteria to give me the best results. Average AC IR rise was 0.11mΩ with an average V drop of 0.0335V which translates to an average voltage drop per day of 0.000162V.
So as far as I am concerned for good meaningful data multiple AC IR and voltage tests should be done throughout the cells harvesting and testing process so you can filter out the best of the best cells for your build.

Wolf
 
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