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DC IR VS AC 1kH IR measurements
#1
Discussion about the benefits of DC IR testing versus  AC 1kH IR ( Impedance) testing.
2 wire vs 4 wire kelvin etc.

Primer of discussion copied from another thread.

gauss163 Wrote:

Quote:But iirc the Opus is doing a longer time DC IR test (vs. 1ms AC). So it is also measuring some non-ohmic parts of the IR, i.e. additional components of the cell's IR that don't behave linearly as in Ohm's Law and take time longer than 1ms to ramp up to steady-state values (e.g. charge transfer and diffusion processes).  This is why at the end of (dis)charge we see both an instantaneous voltage change (from the ohmic part of the IR - which behaves like a resistor),  followed by a much slower exponentially decaying voltage change that asymptotically approaches the steady-state resting voltage (from the non-ohmic IR, which takes a long time (hours) to ramp down as various elecrochemical processes converge to equilibrium).

In many cases it is the non-ohmic part of the IR (vs. capacity degradation) that ends up limiting the lifetime of the cell because it can degrade at 10 times the rate of the ohmic part, and this greatly drags down the voltage under load (esp. for high-current loads).  Whether or nor this is a limiting factor depends on what type of loads you put on the battery. If your device only uses short pulses (e.g. a jump starter or vape) then the non-ohmic IR plays little role since the discharges aren't long enough for it to kick in. OTOH if you are doing longer time discharges without big spikes then the non-ohmic IR plays a big role.  Some loads are a mixture of both and require more complex analysis (e.g. to see if a constant load plus occasional spike drags the voltage low enough for long enough time to trigger undervoltage protection).
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#2
Here is an interesting read about lithium ion battery impedance.  They have a resistive and inductive component.  If I recall there isn't a capacities component.

https://www.google.com/url?sa=t&rct=j&q=...Ea1f8R86be


Z is the impedance.  The mΩ reading we get is actually Impedance so you can use the same formula as above. .... Just most people understand that they have been checking resistance so I used in the previous formula R1.R2 etc.
Please check out this article --> https://www.allaboutcircuits.com/textboo...-circuits/

I'll be honest it's been 25yrs since I studied battery theory and used superposition theorem but that's what were getting into.
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#3
(07-06-2020, 11:00 PM)Bubba Wrote: Here is an interesting read about lithium ion battery impedance.  They have a resistive and inductive component.  If I recall there isn't a capacities component.................... it's been 25yrs since I studied battery theory and used superposition theorem but that's what were getting into.
In the vernacular of Spock "Fascinating"


gauss163,

First off please be aware that I find your devotion to your DC IR measuring cause very refreshing. I myself have absolutely no quarrel with the idea that there may be a better way to determine the longevity and or quality of a USED 18650 Li-Ion cell. For that matter of fact any Li chemistry or package.
I am also not opposed to spending 10, 20 or 30 seconds to do a test if in the long run it provides me with accurate, definitive and consistent results by which I can make an educated choice whether I can use this cell or not. At this time I have not found such a tester commercially available, reasonably priced, and is accurate and consistent which also does not make me jump trough hoops, hang from chandeliers, and employ math skills the likes of a Mars landing ( oh wait one of those didn't work out to well) to get proper results.

At this time the 1 kHz 4 wire kelvin method, which is incorporated into the YR1030, and RC3563 style testers, has been the most stable and consistent standard that I have gone by and the results show it. There are more than a couple of members here who have incorporated this method and found it to be fruitful and has given them good results.
As they say the proof is in the pudding.

As you know if you have looked at my Harvested Cell Analysis sheet I measured the "DC IR" of the testers that allowed a relatively quick result and recorded it. And yes I did spin the cell and made sure the contacts were as clean as possible.
The problem was that the results where not consistent and consistency is important when establishing a rule to go by. Not lets try it 3 or 4 times and take an average. It's like a man with 3 or 4 watches he never really knows what the exact time is. I for one have a bad habit of not accepting close enough. For me I have to have a trust in a piece of equipment that it will give me accurate results always.
Hence I have 3 Fluke bench-top DMs.  8810A, 8842A and 8808A. If all 3 agree to a measurement I know its right.
The consistency of the 1 kHz 4 wire method is unmatched as I can take a cell measurement put it aside for a month and measure it again and it will be within 0.05mΩ of the original reading. Providing the cell is of reasonable quality to begin with.

Cells with a low or 0% SOC will have a higher IR and as the cell approaches 100%SOC the IR will drop about  3% to 10% and after a 30 or so day rest just a little more.
See sheet.


Now these are real life results on USED cells. I'm not about to brake out my Algebra, Calculus, or other means of math books to determine if a cell is good or not. I'm too old for that I will leave that to the young and eager ones.
I have read a bunch of studies on this subject till my eyes have glazed over and I have to get back to reality and say to myself KISS. (Keep it simple stupid)
We are mostly laymen here not collage professors looking for a sponsorship from some agency on how to test LI-ion batteries with outcomes skewed to some  special interest group or manufacturer. 

Now all that being said I am intrigued as to how to go about measuring this "DC IR". I understand that it is a simple ohms law calculation measuring the voltage drop on a battery with a known value resistor as a load. The issue that I have is that the cell to be tested needs to have some SOC to produce a V drop to begin with.
Where do we start? 25% 50% 75% 100%? Do we need to measure IR at each step of the way to determine a cell being good?

So my next adventure will be to figure out a "simple" micro controller project to emulate a 4 wire DC IR tester that is easy to use and has consistent results.
I have found some sketches and schematics and will peruse this in my spare time. 

Wolf
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#4
The IR at 1kHz is more inline with the DI/DT of a fault current. i.e. the effective impedance offered up on an initial short circuit compared to the steady state DC short current which is then chemically constrained at a lower reaction rate.

Is the 1kHz suare wave or sinusoidal ? For kHz loading simple PWM should be ok and more representative of inverter PWM load switching a battery pack might see at 20-30kHz.

I do wonder if the IR at different frequencies and loading would actually give the real chemical signature (and state) of the cell
Volts no load
Load current
Frequency of loading (PWM)
Effective IR

One test would be to load a cell and then sweep the frequency up from say 500Hz to 30kHz and see if there are any specific deviations. Issue here is you then need to look at any stray inducance and capacitance in the setup. Run a dummy cell (power supply) for one test to get a baseline...

The IR does change through the charge state and the makeup of the reactions are different across the voltage range. So, one theory is that if only some of the chemicals in the cell (electrolyte mix) have degraded the cell may be still be fine in one voltage range but not the other.

Some really interesting test results to look forward to !!
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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#5
I'm curious with real world in mind, eg cell in service, lightly loaded, then another appliance switched on.
Thinking of the no load & then loaded cell, ion/chemistry startup time (if that's a thing?)
Maybe fast switch on of a load on the scope might help show this? Might play with this here & see if I can find anything!
Also, our inverters tend to draw a 100Hz or 120Hz "half-sine" current depending if we're 50 or 60Hz.
Testing IR at these frequencies might be helpful?
Inverter switching frequency(eg 25kHz) would be another one to look at although any half decent inverter input caps should stop much of this.
Another way might be DC step load testing, eg you load the cell with a constant current then step it up/down but not to zero.
Running off solar, DIY & electronics fan :-)
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#6
(07-07-2020, 04:37 AM)Redpacket Wrote: I'm curious with real world in mind, eg cell in service, lightly loaded, then another appliance switched on.
Thinking of the no load & then loaded cell, ion/chemistry startup time (if that's a thing?)
Maybe fast switch on of a load on the scope might help show this? Might play with this here & see if I can find anything!
Also, our inverters tend to draw a 100Hz or 120Hz "half-sine" current depending if we're 50 or 60Hz.
Testing IR at these frequencies might be helpful?
Inverter switching frequency(eg 25kHz) would be another one to look at although any half decent inverter input caps should stop much of this.
Another way might be DC step load testing, eg you load the cell with a constant current then step it up/down but not to zero.

Harmonics ... can get complicated very fast if your not careful.
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#7
The batteries will see some of the HF switching, with any LF based inverter the caps are still just a parallel store to the battery so the DI/DT will be shared between caps and battery. The IR of the capacitors then has a consideration. HF types see a lower average draw because the capacitors / buffer are charged through the wave.

The way I think of a battery is just a massive sheet and at rest the whole of the sheet has a reaction potential for high current delivery, then when a current flows some of the area is depleted and takes a relatively large time to replenish the surface. So, the iniitial very brief current delivery capability of a cell when shorted out will be higher, but brief.

So when is the testing to start "Spock, Do something !" Tongue
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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#8
(07-07-2020, 01:49 PM)completelycharged Wrote: So when is the testing to start "Spock, Do something !" Tongue

Waite where did I put my Science officers uniform? Its got to be here somewhere. Ah there it is and look it still fits.............

I was thinking............. I already have the hardware and sketch for testing cells in parallel so why not modify it a bit.

Build a board with 1 proper 4 wire cell holder a 1Ω 1% resistor (possibly more to toggle a heavy load)  several IFR520 MOSes to control the load(s) in a variety of forms. An INA260 Current sensor and an ADS1115 for 4 wire sense AD conversion. A V divider to make it palatable for a 3.3V ESP32 writing data to influx (may have node red as an intermediary for instant data) once a second. Hmm Fascinating................
And I swore I wasn't going to pull out my math books darn you gauss163

Stay tuned

Wolf
PS I do have a day job but no night life Tongue
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#9
Have a look at a servo driver module (I2C) to use for the PWM output (to switch FET) as this will give a lot more reliable and stable output wile eliminating the need for separate timing loops. Loads of code already exists for controlling them as well.

Could always end up using the PWM to vary the attached load at a later date for other types of testing (measuring voltage drop profile at different loading at different charge state)

The whole setup may end up yelling "canna take it any more Captain !"
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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#10
(07-07-2020, 03:24 PM)completelycharged Wrote: .............. servo driver module (I2C) to use for the PWM output (to switch FET) ...........................
The whole setup may end up yelling "canna take it any more Captain !"

LOL ordered.
Wolf
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