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DC IR VS AC 1kH IR measurements
#21
gauss163 : Would a cell with a 1kHz load have more cycle life than a purely DC load (assuming the same energy per cycle) ?
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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#22
(07-08-2020, 03:21 AM)Wolf Wrote: [...] 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.
 

Ok, let's see if we can sync your observations with said theory. First your graph shows that cells with very high AC IR have very bad capacity - which is as expected (same would be true for DC IR). Most of the other cells in that chart are all all close to 100% SOH and have close to the minimal AC IR, again as expected. 

There are only a few cells listed with SOH between these extremes - two about 83% and one at 77% SOH. But they have the same AC IR as other cells near 100% SOH so AC IR can't detect these significant SOH differences. But likely the DC IR will detect it. But I can't verify that since I can't find the DC IR values anywhere (and it seems the cell numbers are incorrect so I can't easily look them up).  

To get better intuition it would help to have a wider distribution of SOH that includes more of these discrepancies.

(07-08-2020, 04:37 AM)completelycharged Wrote: gauss163 : Would a cell with a 1kHz load have more cycle life than a purely DC load (assuming the same energy per cycle) ?

I don't recall any studies that compare such. Cycle life studies are very expensive so they are (relatively) few and far between.

There are some studies on pulse-charging but it's not clear if they translate to pulsed loads, e.g. below

From Capacity fade studies of Lithium Ion cells, by B. Popov et al.

Conclusions

• Pulse charging increases the discharge capacity of the cell.

• Pulse charging decreases the cell impedance by increasing the utilization of Li.

• Discharge capacities evaluated at different cycle numbers indicated that pulse charged batteries retain more capacity.

From The effects of pulse charging on cycling characteristics of commercial lithium-ion batteries by Jun Li et al.

The results show that pulse charging is helpful in eliminating concentration polarization, increasing the power transfer rate, and lowering charge time by removing the need for constant voltage charging in the conventional protocol ... pulse charging maintains the stability of the LiCoO2 cathode better than dc charging and inhibits the increase in the thickness of the passive film on the anode during cycling

You can find more recent work by chasing links to the above.
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#23
Why do they say IR does not change over time/many cycles ?
https://batteryuniversity.com/learn/arti...resistance
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#24
Because its true on its normal span of cycles/lifecycle. IR do not change much during a normal life. You will also see when digging deeper that most places never talk about what happens to a cell when you drastically extend the cycle life of a battery Smile


BUT with that said and we all know that this is a generic saying based on tests and theories. I would say that Batterylife also do flatten alot of the information on how things work around batteries. The information on that site works well as long as you only rely on that information only.
If you start reading scientific papers around this and start to test it heavily you will see that they left out details Smile

Something you need to consider to when reading paper is the result. How did they come up with the result? Many papers dont have a big enough test range for it to even be conclusive. Testing 10 cells give nothing of real life more than an indication.
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#25
I thought so also, thanks for confirming.
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#26
(07-08-2020, 05:27 AM)gauss163 Wrote: There are only a few cells listed with SOH between these extremes - two about 83% and one at 77% SOH. But they have the same AC IR as other cells near 100% SOH so AC IR can't detect these significant SOH differences.

You are talking about Cell #  1210, 1220, and 1245

Actually if you took the time to dig through my excel sheet you would see that out of 171 tested CGR18650CGs 3 of them "failed" with a "normal AC IR"  Oddly enough the Liitokala and Opus didn't think the DC IR was that bad. So what gives?
Maybe a tester malfunction who knows but as far as statistics is concerned these 3 blips are noise. when you look at the whole picture
as in the pivot table it becomes a bit clearer or if you look at the whole 171 count. Posted here for your viewing pleasure.


But likely the DC IR will detect it. But I can't verify that since I can't find the DC IR values anywhere (and it seems the cell numbers are incorrect so I can't easily look them up).  
The cell numbers are correct and you can look them up easily enough. Additionally the DC IR is posted there. A little mouse hover over the chart will reveal the cell number and then you can filter the sheet with those numbers. Simple really if you know how to use excel.
If you are using it in google sheets forget about it download the sheet and open it in excel. 

To get better intuition it would help to have a wider distribution of SOH that includes more of these discrepancies.
Plenty of discrepancies in a sheet with over 6000 cells.............. LOL

So that brings me to my next point.
When are we going to see your testing procedures, results and numbers, not just spewing studies and academia. We are mainly DIY here not academics pulling from this study or that study. Most of us have actually had to work on and fix the engineering fopas made by academics and engineers.
We are where the rubber hits the road, true life experience with hundreds of thousands if not millions of cells in use. Most of them have not even been checked for AC IR let alone DC IR.
IIRC this will be the 3rd time I have asked you to share your work with us but I have not seen any........................ crickets................

Here is the start of my DC IR tester. Ill show you mine if you show me yours Tongue
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#27
(07-08-2020, 06:54 AM)daromer Wrote: Because its true on its normal span of cycles/lifecycle. IR do not change much during a normal life. You will also see when digging deeper that most places never talk about what happens to a cell when you drastically extend the cycle life of a battery Smile


BUT with that said and we all know that this is a generic saying based on tests and theories. I would say that Batterylife also do flatten alot of the information on how things work around batteries. The information on that site works well as long as you only rely on that information only.
If you start reading scientific papers around this and start to test it heavily you will see that they left out details Smile

Something you need to consider to when reading paper is the result. How did they come up with the result? Many papers dont have a big enough test range for it to even be conclusive. Testing 10 cells give nothing of real life more than an indication.

I too found that most of the book is too vague  https://www.scribd.com/document/36910556...le-devices
I don't think the author did their own tests.  There is too much information missing to support the conclusions they arrive at.  What did the waveforms look like?  What equipment was used?
Is this based on 1 cell or the results of many?
What  What occurred with the samples at 1kHz... they jumped.

My honest feeling is that if this information was accurate you would see more papers and data about using lower freq. to determine battery health by now.  Published in 2013 the book is 7yrs old and there have been advances.  On the surface the book looks good, but it brushes over things and doesn't seem to provide proper reference to the backing data.  It's a book to make $ lacking engineering support and focus.
I did find the BIT-BMS which uses low freq. but the low freq is only used for SOC% and 1kHz is still used to determine if cells are good or not.
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#28
The reason you cant determine soh of a battery via for instance IR is because it isnt accurate enough. If that would have been the case guess what Big Grin

With that said you can get some kind of in indication of "Older" cells. This is what Wolf and co have shown. Same as can be seen in the whitepapers out htere if you read a coupld and add upp the information.

This also relates to that degradation beyond 80-60% SOH is also a bit vague. Not in terms of tests but in terms of how the cells behave. There are a couple of REALLY good articles out there showing it.

I have also done my fair share talking to some bigger manufactures about above topic and have gotten that confirmed. Its an very interesting topic. Especially if you like statistics and analysed data. Because you can with the numbers turn them in such a direction that they fit the goal of your report Big Grin

bla bla blah.. Enough ramble from me Tongue
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#29
(07-08-2020, 01:11 PM)daromer Wrote: ................... Especially if you like statistics and analysed data. Because you can with the numbers turn them in such a direction that they fit the goal of your report Big Grin

bla bla blah.. Enough ramble from me Tongue

Exactly what I said.

Quote:We are mostly laymen here not college 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.


Eh you can rabble its allowed Big Grin

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#30
(07-08-2020, 12:38 PM)Wolf Wrote: There are only a few cells listed with SOH between these extremes - two about 83% and one at 77% SOH. But they have the same AC IR as other cells near 100% SOH so AC IR can't detect these significant SOH differences.

You are talking about Cell #  1210, 1220, and 1245
Actually if you took the time to dig through my excel sheet you would see that out of 171 tested CGR18650CGs 3 of them "failed" with a "normal AC IR"  Oddly enough the Liitokala and Opus didn't think the DC IR was that bad. So what gives? [...]

Your prior post gave only (excerpted) images. I need the link to the spreadsheet with the DC IR values, and also the link to the posted graph with IR vs remaining capacity. 

I'll have to figure out how to use google sheets since I don't have excel here. It's difficult to make any conjectures until I can see more data points.  

The DC IR should reveal more, but there could also be other factors that come into play. Are all the cells from the same source? Are all cells used or are some used and some new-old-stock?

(07-08-2020, 12:38 PM)Wolf Wrote: So that brings me to my next point. When are we going to see your testing procedures, results and numbers [...]

I don't build powerwalls so I have no further data on that. But I have extensive knowledge of Li-ion batteries from prior (professional) work, so I may be able to help in various ways.
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