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DC IR VS AC 1kH IR measurements
#51
Well the DC IR experiment continues. The prototype is functional and giving good results.
With a 1Ω resistor the numbers look like this on a Sony US18650VT4

RC3563 1kHz AC IR 19.247mΩ  4.209 V

The prototype DC IR Tester

V Open circuit  4.208
V Loaded circuit  4.043
mV Voltage Drop  0.165
mA Circuit draw  3422.50
mΩ  DC IR  48.31861


Stay tuned  Wolf
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#52
So I did a few simple tests on an LGABC41865 cell from a laptop pack.
I did DC load steps.
I don't have a nice test gear as Wolf & some of you guys & it wasn't an automated test but here goes:
Meter A = Protech 506 Multimeter
Meter B = YR1030+ 4wire 1kHz resistance meter
Resistors, 2x 47ohm/5W wire wound measured with meter A = 47.0, 47.2
Resistors, 2x 6.8 ohm/5W wire wound measured with meter B = 6.74, 6.79

Temp =~18 degC
The cell had approx 1cm of typical spot welds straps attached each end which likely added a few m.ohms
Cell volts resting before test: 3.926V
Cell measured approx 50.1 m.ohms with B at 1kHz before tests
Allowed approx 5 sec to stabilize at each load point
Resistors re-checked after tests, no significant change eg due to heating, etc

Test 1:
Load with 1x 47 ohm resistor, measured volts with A, = 3.917V = 83.34mA
Add 2nd 47 ohm resistor, measure volts with A, =3.910V = 166.03mA
So Cell Resistance = delta V over delta A = 7mV/82.69mA = approx 84.6 m.ohms
Cell at rest again recovered to approx 3.924V ~30 sec after test

Test 2: (approx 5 mins after test 1)
Load with 1x 6.8 ohm resistor, measured volts with A, = 3.856V = 570.4mA
Add 2nd 6.8 ohm resistor, measure volts with A, =3.807V = 1.1263A
So Cell Resistance = delta V over delta A = 49mV/555.9mA = approx 88.1 m.ohms
Cell at rest again recovered to approx 3.920V ~30 sec after test

I repeated the tests on the same cell & got similar numbers
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#53
All right here is where we are so far.
I have managed to cobble together some code for this DC IR tester.
Here was my theory.
Build a 4wire voltage tester with an Amp sensor. Record the V open, V drop, mA draw and calculate the DC IR
as in (V open - V drop) / ma = R
I built this tester using an ESP32 with an ADS1115 V sensor and a INA260 Amp sensor. A 4 wire cell holding fixture and a 1Ω 100W resistor.
The function is as follows:
A charged cell is inserted into the fixture and the ESP32 is in a while loop waiting for a push button to exit it and run the main loop.
The main loop measures the V open saves that and engages the MOSFET for 1 second while the 2 sensors record V drop and mA.
All gets spit out in real time to excel using PLX-DAQ.
Once the button is released the while loop resumes.
I have tested several cells and the results are posted below.
The trace of the charts goes from green stripe to green stripe north and south of the green stripes is noise.
For some reason the MOSFET engages for a split second when the button is pushed so the first readings (north of the green line) are right but wrong. Huh
I have no idea why it does this. Maybe someone can give me a pointer there. 
The averages are calculated between the white lines including the blues with results to the right of the charts. Also included is the cell model AC IR and V measurements prior to inserting the cell.
I have run the same cells several times and the results are almost always identical within a couple of mΩs.
One interesting thing is the traces are somewhat different for each cell but same part numbers seem to be similar.
We all understand that DC IR is going to be different and higher and it appears that cells I measured with AC IR and claim in my cheat sheet to be marginal certainly have a high DC IR.
This is the best I have come up with in a quick easy DC IR tester that is adjustable in the code to pretty much anything you want as far as time and measurement parameters is concerned. I'm sure there is much that can be improved with the sketch so if anyone wants to help I will gladly share.
So much more to study and learn..............
Oh yea that lonely led on the left is lit to tell me the while loop is running and the system is ready for a test.
OK pictures as promised.
The Tester:

The Charts:



Wolf
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#54
Glad to see you are making progress.  The wide variation between the AC IR and DC IR gives us something to ponder. Is it correlated to capacity? (please list the capacity too, and the discharge rate and termination). 

Did you test any of the prior mentioned cells where AC IR was ambiguous, i.e. some cells with close AC IR had both good and bad capacity?  Likely a DC IR test would help to further discern the bad guys.
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#55
(07-24-2020, 03:47 AM)gauss163 Wrote: Glad to see you are making progress.  The wide variation between the AC IR and DC IR gives us something to ponder. Is it correlated to capacity? (please list the capacity too, and the discharge rate and termination). 

Did you test any of the prior mentioned cells where AC IR was ambiguous, i.e. some cells with close AC IR had both good and bad capacity?  Likely a DC IR test would help to further discern the bad guys.

Progress yes thank you I'm doing the best I can with my limited knowledge.
The cells tested were a random number of cells that I have just sitting around for just such testing purposes.
At this point it was just to show how the tester works and what information can be gleaned from its results.
Worth mentioning is, that the placement of either a 4Ω or 1Ω resistor as the load value has very little influence on the outcome as initially observed.
I would have both of them hooked to the board with separate MOSFETs but my coding skills are not there yet so I have decided to stick with the 1Ω.
It is easily changed manually if necessary.
Are the results telling us anything? IDK yet. I do know they are consistent for each cell. I have tested several cell multiple times and the results are always very close.
I will redo the tests and post those charts also.
When the time permits I have brand new cells and a bunch of tool and medical packs to break apart and start a sheet on this testing procedure
taking each individual cell from pack liberation to potential healthy cell.

@gauss163
"Is it correlated to capacity" I don't believe so and yes I intend to gather all this data with cells I have never tested.
As far as close AC IR between same manufacturer part numbers I have never had a bad cell with proper AC IR, i.e. if the AC IR was 45mΩ on 1 cell and 44mΩ on another both cells would usually be within ~100mAh of each other.
Also my AC IR cheat sheet has never let me down. If a cell is at the margin of the upper limit it may be a crap shoot but if the cell is well within the range you are 99% guaranteed to get a good cell. 
As I test these cells coming out of these packs it will be interesting.
The new sheet will be very thorough recording all the pertinent measurements for these tests. 
Unfortunately I am only 1 person and do not have a lab other than what you see.
Also I still work to make a living (bummer) as I would love to do this all day long.
Additionally I still need to finish my second 14s80p pack........

Stay tuned
Wolf
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#56
(07-08-2020, 06:54 AM)daromer Wrote:
(07-08-2020, 06:03 AM)Overmind Wrote: Why do they say IR does not change over time/many cycles ?
https://batteryuniversity.com/learn/arti...resistance

Because its true on its normal span of cycles/lifecycle. IR do not change much during a normal life. [...] 

As is clear from prior posts here, AC IR does in fact change during normal cycling, but the change is much smaller than for DC IR, because DC includes additional non-ohmic components, e.g. see post #43, and post #12 (and it is also implicit in wolf's data). That's why both IR measures are used to test battery health.

As I warned before: info at BatteryUniversity is highly unreliable - don't trust it.
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#57
Could do with this fancy looking 4 wire cell holder too Wink

Makes loading cells into the tester soo smooth Smile terminals pivot around a gear and engage the 4wire pickups on the battery ends.. ahhh

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#58
Here is an update on DC IR VS AC IR.
The tester I built seems to be working quite well. I have tested ~100 cells so far and have all this info recorded on this sheet.
https://drive.google.com/file/d/1w1s3S6Z...sp=sharing

Synopsis of the results. (≈) = average of all cells per pack
 
First pack was a medical pack with Moli ICR-18650H  (2100mAh) cells.
The AC IR was so high I would have never even thought about using these cells but for sake of the experiment I did. My preferred IR for this cell was ≤50mΩ these came through at ≈ 78mΩ.
The Voltage was decent at ≈ 3.7V DC IR was ≈ 334mΩ. Woah that was high is my tester working right? Tested the cells and their SOH results, after C/D/C was ≈ 28%. After charge AC IR actually went up ≈ 3.6% and DC IR went slightly down ≈ 6.8%

 
Second pack was another medical pack with Sanyo NCR18650BF (3200mAh) cells.
AC IR was reasonable at ≈ 41mΩ unfortunately I was unable to find a factory spec on AC IR so I assumed it was OK. Voltage was a little low on the cells ≈2.3V. Checked DC IR at that level and found it to be ≈240mΩ. Recovery Charge at 100mA to 150mA to 3V and then full charge. SOH results, after C/D/C ≈ 100%. AC IR dropped a bit ≈ 4.2% and DC IR dropped dramatically which is to be expected ≈ 64%.

 
Third pack was a Hoover Commercial 40V pack with Samsung INR18650-30Q (3000mAh) cells.
AC IR was ≈ 12.6mΩ. Voltage was ≈ 3.8V except for 4 cells which were at ≈ 0.39V. Probably the cause of the demise of the pack. DC IR was not done (on the 4 low V cells) as there was no voltage to drop. AC IR was higher than the others but still below factory spec (≤18mΩ) at ≈ 13.3mΩ. Nevertheless recovery charge at 50mA to 3V and they were tested with the others. DC IR on the other cells was ≈ 26.3mΩ. Wow that’s low. So I started looking at spec sheets and lo and behold I found a spec sheet for the Samsung INR18650-30Q listing AC IR and DC IR. To my surprise the DC IR that I measured matched the manufactures spec at ≤30mΩ. I was feeling good about my tester.
SOH results, after C/D/C, was ≈ 100% including the 4 low V cells. After charge AC IR dropped slightly ≈ 2.7% DC IR dropped slightly also ≈ 3.1%.
 

Fourth pack was another medical pack with Moli ICR-18650K cells. Manufactures AC IR states ≤80mΩ but in my experience and my cheat sheet ≤70mΩ is preferred and a max of 79mΩ although that max may need to be revised in my cheat sheet. AC IR came in at ≈ 85.5mΩ Voltage was a respectable ≈ 3.8V and DC IR was ≈ 303mΩ. SOH after C/D/C was 42%. After charge AC IR decreased ≈ 1% and DC IR decreased ≈ 8.5%.

 
Fifth pack was an EGO power mower pack with Sanyo UR18650RX (1950mAh) cells.
Manufactures AC IR <25mΩ.  AC IR was ≈ 12.1mΩ Voltage respectable ≈ 3.7V and DC IR was ≈ 27.6mΩ. SOH after C/D/C was 103% (skewed because all where tested with OPUS). AC IR dropped negligible by ≈ 0.02%. DC IR dropped by ≈ 9.35%
 

Sixth set of cells did not come from a pack but some cells I have collected for a 12V battery.
Picked 12 random cells out of the 40+ I have.
The set was comprised of 12 Samsung INR18650-20Q (2000mAh) cells. Manufactures AC IR spec was ≤18mΩ. My cheat sheet was at ≤25mΩ and up to 35mΩ with a ? Not enough data.
AC IR was ≈ 25mΩ Voltage was ≈ 4.08V and DC IR was ≈ 58.9mΩ. SOH after C/D/C was ≈ 97%
AC IR dropped ≈ 1.5% and DC IR dropped ≈ 8.1%

 
Seventh pack HP supplementary 12 cell battery I have had for a while. (I have 20 more of them)
But knowing they were fitted with 2200mAh cells I didn’t take them apart yet. Nevertheless now was the time for the experiment.
The pack had 12 Samsung ICR18650-22F (2200mAh) cells with an AC IR of ≈ 58.4mΩ 2.48V and a staggering DC IR of ≈ 291.7mΩ. My cheat sheet claimed preferred status of ≤65mΩ and up to 75mΩ. SOH after C/D/C was ≈ 97% (cheat sheet was right as far as AC IR was concerned) some of the slightly higher V cells had lower DC IR. AC IR dropped by ≈ 1% and DC IR dropped by ≈ 61.6%

And just for kicks here is a screen capture of the DC IR results and how they are calculated. This happens to be cell #0099 1st DC IR Test after C/D/C.


 
Please feel free to download or view this sheet for further study if interested.
File is attached
Several observations and a conclusion.
To demonstrate the validity and consistency of my DC IR tester, the after C/D/C DC IR test was done twice on the same cell. The differences between the 1st and 2nd test are marginal at the least.
So as far as consistent results yes the tester is consistent. As is the RC3563 AC IR tester.
As far as AC IR is concerned it should be the first touch of the cell to determine the feasibility and the potential health of the cell. The advantage of AC IR is that the cell can be tested at a lower voltage and still give accurate results. The same cannot be said for DC IR the cell needs to be charged to at least 4.xx V for a somewhat reasonable result.
Commercial testers IR measurements (OPUS SKYRC etc.) are haphazard at best. At times they are close other times they are in left field.
DC IR is a neat thing to investigate and for sure can indicate a bad cell but the cell has to be charged to be able to determine that. Well if you don’t check anything on the cell and C/D/C it and come up with a SOH of 23% I don’t need to be a rocket scientist to determine this cell is N/G.
On the other hand take the cell before a C/D/C cycle, get an AC IR reading of mΩs that is out of spec then there is no need to go any further with the cell wasting your time on an underperforming cell.
Additionally DC IR at the initial cells harvest state is just not possible most of the time as the SOC of the cells we harvest mostly are at storage levels or below.


So my conclusion follows this presentation done in February, 13, 2020
https://fhi.nl/app/uploads/sites/74/2020...ronica.pdf
 
Conclusions
• Low frequency AC-IR is a valid alternative for DC-IR measurement
• AC-IR reduces measurement time dramatically
• Possible to measure resistance of 1mΩ or less accurately
• 4-terminal pair test leads required to measure low resistance with AC-IR
 
I could not have said it better myself.

Additional reading: https://www.electronicdesign.com/technol...mion-cells
 
Any questions please feel free to ask and comment.
Wolf
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#59
(08-04-2020, 01:31 PM)Araknid Wrote: Could do with this fancy looking 4 wire cell holder too Wink

 ahhh

Tease......
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#60
That's some awesome work there.
I guessing the DC IR results also followed the discharge current rating for the cell?
Ie it would confirm high current cells would have a lower DC IR than say your average laptop cell?
A 1ohm load would be quite a high load for some cells right?
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