Cell Internal Resistance

ChrisD5710

Active member
Joined
Sep 13, 2017
Messages
162
This info I provide here is without warranty, You can read it, You canthink about it and make your own conclusions, or You can simply trash it and think that this crazy Dane should be excluded from posting here. You be thejudge. This is my first attempt so please do not be too harsh in Your comments.

This post is about cell internal resistance, and I provide this info because it seems to me that someof You areunsure of what these figures mean.

Cell internal resistance,It is there on the Opus and Imax testers,so we measure it, discuss it, but first problem is thatwe can not seem toget coherent readings, they go up and down for each time we try, and they do not match the values in the Opus and ImaxManuals,so what can we do?

First advice, do not think too much about the readings, it does not tell You anything about the cell condition, unless you get abnormal high readings.

Cells are manufactured with different charge/discharge ratings, besides all thedifferent capacities.

Two cells may havethe same capacity, but thecharge/discharge rating isdifferent.

A gaming laptop will require cells with a higher dischargerating than a standard laptop, sowhenwe take all these packs apart weget cells with different ratings.

The cellwith the smaller discharge rating will show a higher internal resistance andthe cell with the higher discharge rating will show a smaller internal resistance.

This does not mean thatthe cells with the higher internal resistance isfaulty.


A lot of us get different readings each time we try to measure the internal resistance, how come?

Apart from the obvious reasons as bad connections, cell temperature has influence, so testing several times in a row will get you a little lower value, because the cell gets a bit warmer inside and thus produces the power a bit faster. Cell charge state will also alter the readings.

Finally: Internal Resistance, what isreally going on?

Some Manufacturers data sheets shows something about Internal Impedance,

a 1 KHz sine wave is sent into a fully charged cell and the resultant resistanceis measured.

This test cantell somethingabout the internal condition ofthe cell without having to spend several hours counting the cell capacity.

I have not taken an Opus charger apart, but I do not think there is any 1 KHz tone present. They simply put a load on the cell and measure the voltage drop, but it does not tell You about the cell condition, only its discharge rating.

Now, I mustpull myself together and make a 1 KHz sine wave generator and some kind of a test set that can replicate the values stated by the manufacturers.

Finally, Questions are welcome, and I will do my best to provide You a meaningful answer.

ChrisD5710
 

Abarth595

New member
Joined
May 14, 2017
Messages
23
I myself use a dedicated (budget) Impedance Meter.
This gives more consistent readings then the opus. It also deducts the resistance from the wires and the touchprobes.

And indeed, the values are relative. I choose not to use cells which have an IR of >150. Others here on the forum go up to 250.

If I have a value of 80 on the Opus, this impedance meter will end up around 60. So my 150 value will compare to 170 on the Opus.
 

daromer

Moderator
Staff member
Joined
Oct 8, 2016
Messages
5,591
There is very very detailed documents out there on above topic where they combine the IR meassured from 0.1 to 10kHz comparing to testing the capacity in terms of how old they are.

This dokument here is very interesting: https://www.researchgate.net/public..._Battery_Degradation_for_Cell_Life_Assessment


The different readings is mostly due to contact issues if you ask me. Ie between readings and many cells. An Opus can easily diff 100mOhm between 2 tests on same cell. (>100%). Its easy tested by just pushing the cell contact while running the test.
 
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