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DC IR VS AC 1kH IR measurements
#11
On the next generation of tester..... "Make it so...." Big Grin
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#12
Since a picture is worth a thousand words, below is a graphical illustration of the points I emphasized in my remarks quoted by wolf in the first post. The graph yields a vivid visual depiction of the large difference between DC IR vs AC IR.

An AC 1kHz IR meter measures resistance at frequency 10^3 Hz (= 1 ms pulses). In the graph AC IR is the y value on the cycle curves above the rightmost x-axis value (= 10^3). The graph shows this 1kHz AC IR grows very little in 100 cycles - from about 64 to 70 m?. OTOH, the leftmost frequency .01 Hz uses 100s pulses, which are long enough for the IR from charge transder, diffusion etc to ramp up, which pushes the IR up much higher (above 150 m?). Further, these values increase much more, from about 154 to 242 m? after 100 cycles. These correspond to the values measured by DC IR meters (usually for shorter pulses of a few secs, which still yield large values, about 150 to 218 m? via the graph)

By only measuring the AC IR (= purely ohmic component) we are completely missing these other large components of IR. Only by incorporating them do we obtain an accurate idea of how the voltage drops under load during longer time DC discharges (hence the true capacity under load). The AC 1kHz IR yields this information only for very short timed loads (< 1 ms), e.g. for a car jump start pack or vaping. It is essential to assess how these (larger) values evolve too in order to accurately assess battery health. These DC IR values will also help improve other decisions, e.g. they yield more accurate matching of cells when building packs - which leads to longer pack lifetime.



The graph is excerpted from p. 17 of this book: Barsukov and Qian, Battery Power Management for Portable Devices, 2013 (at libgen or scribd or amazon). The authors are leading experts at Texas Instruments, responsible for the design of TI's impedance tracking fuel gauge algorithm - which is very widely used (e.g. in most laptop batteries). It's a good place to learn about internal resistance / impedance and related matters because this information is crucial to information reported by fuel gauges (which includes health) so is discussed at length there. It was written to be accessible to those without any specialized knowledge of battery electrochemistry (it is much more accessible than the brief blurbs prepended to many research papers).
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#13
gauss163 : Nice post, the link does not work for me.

I would be interested in seeing how the curve actually appears at a significant load level at 100Hz to 1kHz. The results do show the chemical boundary quite nice though, around 8 seconds to stabilize.
Questions would be :
1. Was the testing 50% duty cycle of the PWM ? Raises some questions for how 10-1000Hz looks...
2. What is the correlated deviation around 9Hz and just below 1kHz ? Does not seem like resonance (missing harmonics ?).
3. Are the test series at equal spacing or a random sample, i.e. every 10 cycles ? If so why the inconsistent increments (different electrolyte degredation ?)..

Regarding question 1, argumentative, if you can decrease the effective IR by increasing the frequency then why not replace a pure DC connection with a switched 2kHz PWM 50/50 connection ? i.e. hook up all your kit with a 50% duty 2kHz switch ? Maybe the chart is not showing an equal energy per second across the scale ?

Regarding question 2, maybe these correlated deviations (9Hz and 1kHz ?) are the real fingerprints of cells and the state that is revelaed at >10sec load durations... easier way to test ?
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#14
(07-07-2020, 05:34 PM)gauss163 Wrote: Since a picture is worth a thousand words, below is a graphical illustration of the points I emphasized in my remarks quoted by wolf in the first post. The graph yields a vivid visual depiction of the large difference between DC IR vs AC IR.

So in a nutshell, correct me if I am wrong, I/we need to build a tester that is 4 wire and has the capability of loading a battery for 100s (extreme) to get the "chemical reaction" to take place then and only then will we get an accurate DC IR.

Sounds good to me. Got any hardware suggestions and plans?
I am working on building such a device but if you have a head start by all means please do share.

I do have a question though.
Playing the devil's advocate................
So I measure the cells IR with a 1kHz meter and I get an acceptable reading according to my cheat sheet. Plop the battery into an OPUS at a C/D/C rate of 1A and it comes out at 90% SOH. Haven't I basically checked this battery for a high DC IR as I have exercised this battery for ≈7½ to 9 hours for the complete C/D/C cycle and it did not overheat and gave good mAh results.
The same can be said true of a high 1kHz IR reading as I have many examples of those with low mAh results and potential hot running ≥45° C.

The other thing is the time it would take to get these DC IR readings for a substantial powerwall say 2800 cells for a 14s200p wall.
It is hard enough to convince some members that IR is actually an important measurement let alone add another cog into the gear. 

I will have fun with this and more than likely learn some more things. I like that.

One thing I do know though.  I never trust " leading experts at Texas Instruments" (Or any other EXPERTS) especially the ones that are responsible for the design of TI's impedance tracking fuel gauge algorithm - which is very widely used (e.g. in most laptop batteries) As I have had plenty of laptop battery fuel gauge inaccuracies cause my work to disappear.---- Poof-----
These experts always remind me of the microsoft file transfer utility that tell me how long its going to take only to constantly giving me a moving target.

Wolf
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#15
"I will have fun with this and more than likely learn some more things. I like that." +1 on that and keen to learn from the testing results as well.
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#16

(07-08-2020, 12:20 AM)Bubba Wrote:
(07-07-2020, 05:34 PM)gauss163 Wrote:

The graph is excerpted from p. 17 of Barsukov and Qian, Battery Power Management for Portable Devices, 2013.  The authors are leading experts at Texas Instruments, responsible for the design of TI's impedance tracking fuel gauge algorithm - which is very widely used (e.g. in most laptop batteries). It's a good place to learn about internal resistance / impedance and related matters because this information is crucial to information reported by fuel gauges (which includes health) so is discussed at length there. It was written to be accessible to those without any specialized knowledge of battery electrochemistry (it is much more accessible than the brief blurbs prepended to many research papers).


https://www.scribd.com/document/36910556...le-devices

I don't see this chart as useful if you look each cycle is C/10 and the IR change after 100 cycles rises to 240m?  ?  
100 cycles is nothing!  I hope I could get several thousand cycles at that low C rate.
The chart i think is incomplete since they don't list the what the symbols are and what is going on at 1kHz and 10Hz?

My conclusion this graph should have been based on 1000+ cycles and should list % of usable capacity as the trends.
The graph doesn't indicate % loss.

Please someone correct me.
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#17
(07-08-2020, 12:20 AM)Bubba Wrote: https://www.scribd.com/document/36910556...le-devices

I don't see this chart as useful if you look each cycle is C/10 and the IR change after 100 cycles rises to 240mΩ  ?  
100 cycles is nothing!  I hope I could get several thousand cycles at that low C rate.
The chart i think is incomplete since they don't list the what the symbols are and what is going on at 1kHz and 10Hz?

My conclusion this graph should have been based on 1000+ cycles and should list % of usable capacity as the trends.
The graph doesn't indicate % loss.

Such graphs (Bode plots) are not intended to show capacity loss. Rather their purpose is to show how IR (or impedance) depends on load frequency, in order to give a comprehensive view of how a battery responds to diverse signals. There is no need to do further cycles since the points I made above are already quite clear after 100 cycles.

The same shape graphs appear for all Li-ion batteries, though the absolute numbers will depend on chemistry etc. Such graphs are basic tools that are widely used in battery electrochemistry.

For further background on this and related matters see said book, section 1.3.1 p. 12 where they discuss the closely related Nyquist plot of the impedance spectrum, and see the following section where they explain how IR (or impedance) affects the usable capacity.
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#18
(07-08-2020, 01:57 AM)gauss163 Wrote:
(07-08-2020, 12:20 AM)Bubba Wrote: https://www.scribd.com/document/36910556...le-devices

I don't see this chart as useful if you look each cycle is C/10 and the IR change after 100 cycles rises to 240mΩ  ?  
100 cycles is nothing!  I hope I could get several thousand cycles at that low C rate.
The chart i think is incomplete since they don't list the what the symbols are and what is going on at 1kHz and 10Hz?

My conclusion this graph should have been based on 1000+ cycles and should list % of usable capacity as the trends.
The graph doesn't indicate % loss.

Such graphs (Bode plots) are not intended to show capacity loss. Rather their purpose is to show how IR (or impedance) depends on load frequency, in order to give a comprehensive view of how a battery responds to diverse signals. There is no need to do further cycles since the points I made above are already quite clear after 100 cycles.

The same shape graphs appear for all Li-ion batteries, though the absolute numbers will depend on chemistry etc. Such graphs are basic tools that are widely used in battery electrochemistry.

For further background on this and related matters see said book, section 1.3.1 p. 12 where they discuss the closely related Nyquist plot of the impedance spectrum, and see the following section where they explain how IR (or impedance) affects the usable capacity.
So to draw a conclusion from that cart... how the trend seems to be going you will have resistances in several Thousand mΩ   (or in several full ohms).
I would be curious to find out what variability there is in the readings with such wide range as a result of such few cycles.
If someone could find that information it would be useful regarding the 100s test. vs 1kHz  I haven't been able to in a quick search.
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#19
(07-08-2020, 02:31 AM)Bubba Wrote: So to draw a conclusion from that cart... how the trend seems to be going you will have resistances in several Thousand mΩ   (or in several full ohms).

There are many studies that show how IR evolves over the entire life cycle, but that is not the purpose of the above. Rather, it is simply to contrast the large IR differences at low-frequency (DC IR) vs high-frequency (such as AC 1kHz IR).

Of course cells will no longer be usable for most practical purposes long before their IR has a chance to grow to monster values (so no one studies them all the way to such extreme overkill).


(07-08-2020, 02:31 AM)Bubba Wrote: I would be curious to find out what variability there is in the readings with such wide range as a result of such few cycles.
If someone could find that information it would be useful regarding the 100s test. vs 1kHz  I haven't been able to in a quick search.

You can find some further examples of DC IR vs. AC IR in my initial post.
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#20
Excerpt from 1.3.3 Usable Capacity
Therefore, simple modeling assuming fixed R will not give an accurate estimate of usable capacity. Also battery manufacturers often report battery impedance at 1 kHz. This value cannot be used as an estimate for internal resistance at DC conditions, because low frequency impedance (that corresponds to DC conditions) is much higher than that at 1 kHz. Although DC resistance is typically two to three times the 1-kHz resistance for charged Li-ion cells, the ratio is unpredictable for different states of charge, aged cells, or at low temperatures, so it is better to always measure DC resistance directly. Impedance of 1 kHz is useful for detecting catastrophically failed cells during production because of the fast measurement ability afforded and widely available instrumentation. 

I have to slightly disagree with these statements and here is why.
First of, if I measure a cell at 1 kHz I am not trying to ascertain the internal resistance at DC conditions. I am merely running a quick check on the feasibility that this cell is good enough to charge and test.
I am sure the ratio is unpredictable between AC IR and DC IR but I can tell you that AC IR can pretty much predict the SOH of a battery.
These are the same manufacturer cells I have been testing. I'm at 1468 cells recorded so far.
You can see that these cells have a high IR cutoff of around 42mΩ measured with a 1 kHz tester. They are marginal but defiantly degraded.

The lowest IR cells all are good.


Have you even looked at the pivot chart in my harvested cell analysts sheet where you can select a cell manufacturer and mode number and see the IR to capacity correlation? It has over 6000 cells in it from various manufactures and the trend is as clear as crystal.
 

Again I am not opposed to a DC IR test and am planning of building such a tester.
By the way since you are such an advocate of this method please provide some insight on how you do your testing. Have you built a DC IR tester? Or at least show us how you do it.
It would be nice to collaborate on this project,
Also any sheets that you have to show what cells you have tested, your results as far as DC IR measurements, and its correlation to capacity results would be appreciative.

Thanks

Wolf
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