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DC IR VS AC 1kH IR measurements
#61
(08-04-2020, 11:44 PM)Redpacket Wrote: 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?
Yes high discharge or high drain cells such as IMR INR NCA chemistry cells tend to have a low AC or DC IR to begin with. 
ICR chemistry is of course low drain and henceforth inherently has a higher AC and DC IR.
Preliminary results show with my rudimentary tester based on ohms law that the difference between AC and DC IR on a good cell is about 1 to 2.
Meaning that if a cell has 12mΩ of AC IR it will more than likely have a 25mΩ DC IR for a high drain cell.
The same holds true for low drain with an IR of say 50mΩ will have a DC IR of 100mΩ.
A constant 1Ω load would be high for most cells yes but in my case it is only applied for 1 second to get a stable vdrop Voltage.
I have tried 100ms 500ms and all kinds of variables. but the 1 second load seems to be the sweet spot to get a repeatable result.
I can do this test 3 or 4 times on a fully charged good cell and get the same results within a few mΩ every time.
My tester has a 1Ω 100W resistor load that gets put on the battery at 4.1 or so V. Technically that should produce a 4.1 Amp current but it does not as you can see by my discharge chart it is in the neighborhood of 2 .xxx Amps. ( I have verified that with my Fluke meters) The load wire is 10AWG very capable of handling that current. The sense wires are 20AWG although that really doesn't matter as there is no current flow through them.
So the calculation is V(open) - V(load) = V(drop)/mA(draw) = R
Unfortunately as the second paper I linked to basically says that there are no DC IR standards set so how do we know we are on a the right track?
Article published APR 15, 2020. https://www.electronicdesign.com/technol...mion-cells

So I took an educated guess and these are my results. Are they right? I guess so. We are in somewhat uncharted territory and till there is a standard set we just don't know.
One encouraging spec sheet for the 18650-30Q does give us a DC IR value of:

Type Spec.                          (Tentative)                         Typical INR18650-30Q
Initial IR (mΩ DC (10A-1A))       ≤ 30                                      19.94 ± 2
But how was this determined?
My tester seems to hit that nail on the head does that mean it is right?
 ¯\_(ツ)_/¯

Wolf
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#62
(08-04-2020, 06:52 PM)Wolf Wrote: [...] So my conclusion follows this presentation done in February, 13, 2020
https://fhi.nl/app/uploads/sites/74/2020...ronica.pdf
Hioki Wrote:Conclusions
Low frequency AC-IR is a valid alternative for DC-IR measurement  [my emphasis - gauss]
• 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

Oops, it seems you've misread. That conclusion does not support your claim. Rather Hioki's (full) argument emphasizes the same points I've made above and elsewhere about the importance of DC IR as a much more refined measure of health vs. purely ohmic 1 kHZ AC IR measurements. One of the dangers of reading only summary conclusions is that we can miss subtleties abbreviated in a summary. Above it seems you missed the crucial modifier "low frequency" in the key claim that "Low frequency AC-IR is a valid alternative for DC-IR measurement". 

If you read the entire slideshow (or better their white paper) you'll see that the primary point they make is that low frequency (1 Hz) AC IR measurement is closely correlated with DC IR measurement, so it may be used as an equivalent of DC IR (Hioki and others have leveraged this point for many years to market high-end $5000 variable-frequency AC impedance meters). But your $50 AC IR meters cannot perform low-frequency (1 Hz) tests; rather they can only do a fixed high-frequency (1 kHz) test, which cannot measure the longer timescale non-ohmic components (R2 below) of the resistance (major components of degradation). Unfortunately the cell batches in your tests above don't have a wide-enough distribution of health to make this point clear (but I suspect some of your older tests do).

In fact Hioki graphically emphasizes this in the slide preceding the conclusion you quoted - which I have copied below. Just as I showed in prior posts on Nyquist plots, note how as the cell ages the ohmic component R1 (= 1 kHz AC IR) changes very little, but the non-ohmic component R2 changes much more greatly (here DC IR = R1 + R2).
[Image: oRiIE.png]
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#63
I stand corrected.
1Hz AC IR measurement is equivalent to DC IR my mistake.
Time to figure out how to build a 1Hz AC  IR meter.
Although I already have a DC IR meter that is equivalent. I only need to have a fully charged battery to use it.
The other question still needs to be answered though and that is what are the acceptable specifications for individual batteries
that do not have a 1Hz AC IR spec and or a DC IR spec?
We can look at charts on how to measure it all day long but what is a good reading and what is not?
Since there are no standards how do we even know what is right?
There are 2 spec sheets that list DC IR
One is for a Samsung 18650-30Q and the other is for a Samsung INR18650-25R. I have not found any others.
https://eu.nkon.nl/sk/k/30q.pdf
https://www.pdf-archive.com/2015/11/09/d...asheet.pdf
Now that begs the question.
How are we to test batteries from used packs and know that they are reasonably good enough for us to charge and to reuse in our builds?
As far as I am concerned, and from my experience (my cheat sheet) if a high frequency AC IR test can give me an indication of the (somewhat) SOH of a cell
I can at least test the cell and find out if it is any good with an of the shelf charger/tester.
One observation that cannot be disputed though is that if you follow my cheat sheet (measuring cells with a $50.00 1kHz AC IR meter) your success rate with SOH is extremely elevated.

I will continue to try to find other cells ( I have over 3000 cells that have been tested but didn't make the grade)  to round out the test sheet with more marginal cells so that we can get a clearer picture of the relationship between high frequency AC IR and DC IR. 
I do understand that R1+R2 gives us a more total picture of the SOH of the battery given that AC IR is at the same starting point.

Best I can do.

Wolf
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#64
Wolf, just thinking about the current flow & the 2A current (better than 4A IMHO), this could be from the "round trip resistance" of the leads, cell contacts, mosfets switch board, etc, etc?
Might be another 1-2ohms there given the results?
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#65
(08-05-2020, 11:17 PM)Redpacket Wrote: Wolf, just thinking about the current flow & the 2A current (better than 4A IMHO), this could be from the "round trip resistance" of the leads, cell contacts, mosfets switch board, etc, etc?
Might be another 1-2ohms there given the results?
Good Point!
So I did a resistance check with no battery in the holder and the MOSFET on and the resistance was 1.444Ω
I then did a resistance check with my dummy battery installed and came up with 1.432Ω
Also standing resistance  with MOSFET off is 30.701kΩ

4.1 V /1.443Ω = 2.863A getting closer to what the sensor shows.
Wouldn't take a lot of cell contact resistance ~ 0.4Ω to get to 2.2A
Nice observation.
Good thing I didn't take the face value of the resistor as my calculation constant but rather the actual amps.

Wolf
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#66
Curious about the MOSFET resistance, depending on the exact device, I would be looking for "nearly copper wire" on resistance!
I'm thinking this should be < 50 milli ohms.

Off reading is typically higher that 30k (if you fully ground the gate to 0V, doesn't take much Vgs to start a trickle of current). Not that this matters much for these tests!
What's the device part #?

PS, love the work bench pics (active project!) Nice gear too, I'm green with envy!
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#67
(Yesterday, 09:41 AM)Redpacket Wrote: What's the device part #?

IRF520 
https://components101.com/sites/default/...asheet.pdf

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#68
Hmm, that one's not too special there.
Maybe this one instead:
https://au.mouser.com/ProductDetail/Infi...RFP150NPBF
150V, 36 milli ohms Ron....
$2 part :-)
There's lots similar.
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#69
(Yesterday, 01:29 PM)Redpacket Wrote: Hmm, that one's not too special there.
Maybe this one instead:
https://au.mouser.com/ProductDetail/Infi...RFP150NPBF
150V, 36 milli ohms Ron....
$2 part :-)
There's lots similar.
 No not special but what I had available.
IRFP150NPBF Ordered will be here monday.
I assume I can just replace the IRF520  with this one on the board.

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