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Does internal resistance matter?
(11-01-2019, 04:49 AM)Mikethezipper Wrote: Soo.. Wait, did the original post get answered?

Soo........Yes, the original post did get answered "Does internal resistance matter?"
It does.

Granted these explanations are in a different set of posts but............ It proves the point.

I have done a multitude of experiments with cells in parallel with different IR readings and a variety of combinations. Please look through those posts
there is a lot of really good information in there and some very good contributing comments.   

So in the final analysis yes IR matters on how cells discharge and charge. Low drain cells (Higher IR) VS High drain cells (Lower IR).
It does all depend on your application.
I personally will build my packs with low drain >35mΩ to ≤75mΩ cells. Just because the parallel discharge curves show that this is the best combination for efficient use of the cells capacity without overburdening them. 
Throw a bunch of high drain cells (Low IR <35mΩ) in there and the discharge/charge curves change considerably with the cells oscillating between each other and fighting to determine who will supply the demand. Which creates in some circumstances a high discharge rate on some cells causing them to age prematurely compared to other cells. This high discharge rate per cell as been observed at ~.5A draw per cell being most prevalent but not limited to the upper ~(4.2V - 3.9V) range and lower ~(3.5V - 2.8V) range.

So besides being an excellent tool to predetermine if a cell is good enough to spend the time on to test it, It is also a good guide to go by when combining cells in a pack.

Yes internal resistance matters!!

If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 8/7/2019
Total Number of Cells Recorded and processed                 6149
Total Cells required for PowIRwall                                   2856
Total Cells ≥2200mAh, ≥80%, ≥35mΩ, ≤75mΩ, ≥4.12V   2760
For Info Google Drive
Not your average Wolf       
Wolf, I understand that IR is important - however the OP's question wasn't whether IR was an important metric or not - it was whether variance in IR within a pack was important (opening post content vs the title of the thread). Unfortunately he clarified his question several posts in and I'm not sure whether that was answered as it seems the discussion splintered into several directions.

"Question 2: How much variance of IR among the cells should you tolerate (within a pack)? e.g.:
At 1C, all cell's IR should be within 50mOhm from each other
At 2C: all cell's IR should be within 20mOhm from each other"

I understand that it's obviously a bad idea to combine high and low discharge cells together (such as powertool vs laptop batteries) but within the same kind of battery - how much variation is acceptable, and is this dependent on the discharge rate? Logic would say that the higher the C rate, the closer you'd want them... but for powerwall purposes where each individual cell in a pack may be seeing <1/10 of it's rated C value, if all else is equal does the IR matter?

Myself I am only using cells with an IR between 50-80. But what I'm wondering is if it matters if half of my pack is made of cells around 50mOhms and half of them are at 80mOhms -- would that matter? At our powerwall currents it seems like some people don't think it matters.

I have a crap-ton of some lightly used Panasonic NCR18650B cells - so I'm not mixing all sorts of cells. I've ordered in an appropriate IR meter per your suggestion - but from my (understandably inaccurate) Zanflare units, I see cells which have greater than the rated capacity but still have an IR of 100mOhms compared the avg of closer to 50-60mOhms. All cells I have tested are at about 100-200mAh above their rated capacity of 3400mAh. The lowest rated capacity one I've tested so far is at 3450mAH or so. Out of fear I've simply been not including cells with an IR above 80 in my packs - but if from a capacity perspective they are good.... does it matter?

I've already removed any SD or heaters... of which I've had zero. From my recycled laptop batteries I had stuff that was all over the place - so out of fear I just put aside all high IR cells for recycling... but with these basically new cells there are a decent number of them that don't really heat up that much during charging/discharging - but they have an IR that's higher than the average. I would say 70% of them have an IR that is less than 80... but I still have a ton of these cells that are measuring at above 99% capacity but they have an IR that is between 80-120mOhms. What does that indicate? Based on what I saw from your data, a High IR is indicative of it having issues with SD, being a heater, or having a low capacity... which doesn't apply to these nearly new cells. Not sure what to think about it.

The higher the load, the more IR matters. I can see why people say at "powerwall currents" it does not matter, but not all powerwalls and loads are built the same. If you are barely sipping power from the cell, it certainly matters a lot less.

I like building small packs and I have been looking deeper into IR testing under real world loads... I am not using an AC pulse measurement like typically seen in spec sheet measurements or the typical tester, but a resistive DC load test and calculating it manually. Both are valid tests.

Just a a quick test I measured a Samsung 25R
iCharger 4010 = 60 to 65 mOhms
With a 2.05 ohm resistor as a load, I got a IR reading of about 131 mOhms
With a 10 ohm resistor as a load, I got an IR reading of about 63 mOhms

If I was building a "high performance" pack with the intention of drawing a load at 1C or 2C, I would test each cells internal resistance at that C rate and use that IR number for comparison.

Thinking about this from a voltage drop standpoint... it makes sense to me. High drain cell vs low drain laptop cell: In low C rate applications, the voltage drop would be minimal, giving you an IR value with a smaller deviation, this would be a suitable pairing. The same two cells tested at 1C or 2C, the low drain cell will have a significantly higher IR value due to the voltage sag under load. The IR delta between cells would increase showing they would not be a suitable pairing.

While I have all these testers for IR, I am starting to prefer an application specific test method using a DC pulse / load. It just makes more sense to me. There is no hard and fast rule, you just need to look at the application and decide what your tolerances are for these things.
I think the key part is where Wolf said:
"It does all depend on your application"
How about we try to move this discussion into a more practical direction?
Let's say someone is aiming to build a 14s100p system that will have a maximum draw of 500ma per cell. This person has spent 2 years collecting thier 1400 cells that have more than 80% of thier rated capacity, but 280 cells are from power tool (high drain, low IR) packs.
What should this person do?
Use the low IR cells by putting an equal number of them(ie 20 cells) into each 100p pack (ie along with 80 higher IR cells)?
Wait (potentially much longer) to get another 280 laptop pack cells that pass SOH testing (and get rid of 280 good quality cells)?
At what current per cell would your answer change?

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