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Does internal resistance matter?
#1
When you build a battery pack, how much of internal resistance should you tolerate? Let's say if I plan to use the cells at 1C level, would it be a problem if I pair cells that are 100mOhm from each other? How much is too much?
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#2
batteryq
You asked
"how much of internal resistance should you tolerate?"
 
That my friend is the 64 Thousand dollar question.
There are many opinions out there as far as IR is concerned and opinions count as some of them are tied to real life experiences.
There are several on this board that are researching this question with much fervor and are working on an answer.
I am one of those. I have at this time only collected several weeks of data regarding "Heaters" IR, SD and the testing procedure that I have used to come up with some datasets.
This is by no means complete and unfortunately not ready to publish as we do not have enough data as of yet, to come up with a proper conclusion. We are at a theory stage as of yet.
 
So the best I can say at this point is stay tuned and some definitive best practices will come of this I am sure.
 
The best info I can give you for now is that preliminary data shows most (not all) cells which are measured with a Kelvin 4 wire IR reader that are over 70mΩ tend to be heaters esp. the Sanyo brand.
 
That’s all I have for now.
 
Wolf
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#3
Short answer is variable.

The higher the discharge rate, the more IR matters.

You can lower the discharge rate with more cells in parallel. Alternately, experiment with the cells under load to see what type of voltage drop you get and see if it is within an acceptable range for your application..
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#4
Sorry, I just realized I didn't phrase my question properly, there are actually two questions. But I think you guys guessed what I meant:
Question 1: How much resistance is too much at what discharge level? e.g.:
At 1C, IR should be < 300mOhm
At 2C, IR should be < 250mOhm
Question 2: How much variance of IR among the cells should you tolerate? 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 don't fully understand the impact of the 2nd question, what would happen if I pair two cells with very different IR? Is the smaller IR one going to be empty much sooner and then start sucking power from the larger IR cell?
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#5
I would say all between 0 and 200 works fine. Higher resistance do mean they will have issues to cope with the load since u=i*r. As said add more cells and the current per cell is lower and the demand for lower IR decreases too.

First of all if you even plan to go IR route you NEED to have a proper tester. You cannot test with an Opus or Lii tester or equal. You also need to test with a decent current to measure correct drop.

Is it needed? Not for powerwall if you ask me. If you test capacity at max current you will use on a cell you also actually test the IR. because if the cell doesnt get hot then the IR is low enough.

If you want to have proper evidence just read any datasheet or scientific data test out there. There are plenty and i say MANY of them allready done in terms of what IR you should have and plan for. Even IR over time.
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#6
Cool website!

Is there any device that can test the IR for you? I understand the basic concept of how to measure the IR using voltage drop etc, but don't really wan to DIY that thing.

(12-11-2018, 08:06 PM)Wolf Wrote: batteryq
You asked
"how much of internal resistance should you tolerate?"
 
That my friend is the 64 Thousand dollar question.
There are many opinions out there as far as IR is concerned and opinions count as some of them are tied to real life experiences.
There are several on this board that are researching this question with much fervor and are working on an answer.
I am one of those. I have at this time only collected several weeks of data regarding "Heaters" IR, SD and the testing procedure that I have used to come up with some datasets.
This is by no means complete and unfortunately not ready to publish as we do not have enough data as of yet, to come up with a proper conclusion. We are at a theory stage as of yet.
 
So the best I can say at this point is stay tuned and some definitive best practices will come of this I am sure.
 
The best info I can give you for now is that preliminary data shows most (not all) cells which are measured with a Kelvin 4 wire IR reader that are over 70mΩ tend to be heaters esp. the Sanyo brand.
 
That’s all I have for now.
 
Wolf



Can't wait for your testing result!
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#7
Yes, there are several devices. A standard DMM can measure them. The opus and liitokala, and i'm sure others, can test the IR.

The higher the IR, the more difficult it becomes to pull higher amps. Most IR ratings come into place at .5C or more. If you parallel enough together, you won't even get .5C, but closer to .1C. Or, about 300mA per cell. This is so low that IR doesn't have much of an effect.

Think of IR as water, energy required to move as Amp, and your movement as Capacity. If you move slowly through water, you can do it all day long with little energy use. Try to walk fast, and it becomes more difficult. Try to turn, and you burn your energy rather rapidly. As you can see in this real world life size example, IR has a major effect the faster you move, or with the cell the more Amps you try to pull.
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#8
And, fo DIY, You can build an IR Meter or swiper by yourself, given You have an Multimeter with AC mV range, or a simple Oszilloskop Like DSO 138. Look into my Kelvin Thread.
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#9
(12-11-2018, 11:31 PM)Korishan Wrote: Yes, there are several devices. A standard DMM can measure them. The opus and liitokala, and i'm sure others, can test the IR.

The higher the IR, the more difficult it becomes to pull higher amps. Most IR ratings come into place at .5C or more. If you parallel enough together, you won't even get .5C, but closer to .1C. Or, about 300mA per cell. This is so low that IR doesn't have much of an effect.

Think of IR as water, energy required to move as Amp, and your movement as Capacity. If you move slowly through water, you can do it all day long with little energy use. Try to walk fast, and it becomes more difficult. Try to turn, and you burn your energy rather rapidly. As you can see in this real world life size example, IR has a major effect the faster you move, or with the cell the more Amps you try to pull.
I Am sorry to disagree in the fact that Opus ist able to give accurate IR.
More, No Equipment without 4wire Connection give even remotly accurate and stable results, At least Not in the sub 100 mOhm areas.
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#10
Thank you guys for all the great info!

I too find some of the devices which claim to be able to measure IR are not as accurate as one would hoped. I used Liit and zanflare, same battery the IR reported are about 50-100mOhm different, with zanflare always higher than Liit. I am not sure which one to trust.
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