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"Good" IR for common 18650 cells
#1
For those interested in IR and wanting to know what a "good" IR is for various cells I've put together the following table.  This was built from a combination of Wolf, 100kWh Hunter, and my own data.  All IR measurements were taken with a 4-wire Kelvin type AC 1kHz impedance tester, specifically the YR10XX series.  This expands on original work by Wolf, so credit for the idea goes to him.

The values have been derived from a linear best fit line of all measured remaining capacities greater than 70% for each type of cell.  There is still noise within this data, so optimal IR has been defined as the IR which correlates 85% remaining capacity according to the trendline.  Marginal Capacity is defined as the IR which correlates 75% remaining capacity according to the trendline.  This means that you are likely to achieve 80% remaining capacity if you measure the IR to be equal to or less than the optimal IR.  And there's a chance you will achieve it if IR falls between optimal and marginal IR.  If the IR reading is above the marginal IR value you are unlikely to achieve 80% remaining capacity.


The hope is that this will save harvesters a lot of time by allowing them to focus on testing the cells with the greatest chance of success.  Also, if purchasing tested secondhand cells, be sure to ask about IR measurements and ensure they are good!

Thank you very much to Wolf and 100kWh Hunter for sharing their data.  If you like this information and want to contribute, I will accept data collected using reasonably accurate charges (equivalent or better than OPUS) and IR readings from a YR10XX type AC impedance meter or equivalent.  The more data, the better the results.

If you'd like to view the source file it is available on my google drive under "Cell IR Database"

100kwh-hunter, bogptrsn, Wolf And 2 others like this post
Mobilis in Mobili
 
Cell count as of 10/10/2019
234 Cells >2000mAh, >80% Rem. Cap., 14 day resting voltage >4.12V
191 Cells of Everything Else
68 In progress
 
Aiming for 8 cells tested a day
More info on my Google Drive
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#2
Hey Nemo, 

Would love if you could add my log of cells to establish a community based benchmark for the LGDAS318650 cells.
I am using a SM8124A battery tester to measure IR and once i have my IR number i then use a Opus charge test (at 1A) to obtain the mAH of each cell as we shuck through the packs.

Cheers
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#3
Brute-averaging that table one could say that you lose 5% capacity for each 10mOhms. So each 2 extra mOhms from spec value would result in 1% capacity damage.
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#4
(10-21-2019, 08:58 AM)Overmind Wrote: Brute-averaging that table one could say that you lose 5% capacity for each 10mOhms. So each 2 extra mOhms from spec value would result in 1% capacity damage.
 It's probably a good idea for me to include manufacturer's IR in future tables.  I'd have to bring it in to double check but I recall seeing some manufacturer's IR that were way above what turns out to be the marginal IR based on this method.  I don't know how or why that could happen.  Wait for me to update and we can discuss based on actual values.
100kwh-hunter likes this post
Mobilis in Mobili
 
Cell count as of 10/10/2019
234 Cells >2000mAh, >80% Rem. Cap., 14 day resting voltage >4.12V
191 Cells of Everything Else
68 In progress
 
Aiming for 8 cells tested a day
More info on my Google Drive
Reply
#5
I have tested about 7000 cells and have aprox 5000 cells online. When I test (using OPUS) I accept down to 85% of nominal/speced mah with the majority around 93%. I would note 2 things:
1) The posted spreadsheet is exactly in line with my experience - although I agree this is anecdotal I have not way of measuring the accuracy of my OPUSs
2) I'm not sure that X mOhms per capacity (at a couple of %) is meaningful (to me) since my actual operations - battery bank charge/discharge daily over many days - seems to be running perfectly and I'm sure the IRs were a bit here and there regardless of 85% or 93% mah charge test results.

I think that OPUS is a blunt instrument in some ways compared detailed analysis of 1% = 10mOhms. I would put OPUS accuracy more in the range of 5% capacity, 50mOhms, fuzziness overall - something like that.

What do you folks think?
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#6
If you say down to 85% its more like between 70-90% that you accept Smile The Opus is easy +-100mAh and also it generally is a tad high. With that said I dont think its a problem as long as you accept the variance and use it throughout.
iR is more like +-100% on the Opus since it can vary between 50-150mOhm and more just by pressing on the contact.

I think it will work fine for a powerwall as I have stated since many years now Smile
bogptrsn likes this post
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Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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#7
The opus charger was used because I have one, and so do many others.  If you go look at the original data there is lots of fuzziness, but generally speaking the correlation between IR and % capacity remaining is fairly linear to about 70% of capacity, which allows for the linear trendline.  

@ daromer - no doubt, your method of +/-100mAh binning is proven to work.  This exercise is more about reducing the time it takes to collect good cells, and the required investment in chargers.  Using this kind of data you can weed out cells and increase your per charge success rate (finding a good cell) to near 100%.  That means if you have the supply to manage it a person could test enough cells in a year on a single 4 slot charger to build a 14s80p wall.  That significantly lowers the time and gear investment for people just getting into this.
100kwh-hunter likes this post
Mobilis in Mobili
 
Cell count as of 10/10/2019
234 Cells >2000mAh, >80% Rem. Cap., 14 day resting voltage >4.12V
191 Cells of Everything Else
68 In progress
 
Aiming for 8 cells tested a day
More info on my Google Drive
Reply
#8
My experience with OPUS has been quite variable.

I'm dealing with a single type/brand (LGDAS31865), the values i get from the Opus vs the SM8124A meter is wildly different.

Overall the SM8124A meter seems consistent, i kinda wish i added a 'OPUS IR value' and 'SM8124A IR value' in my spreadsheet for comparison purposes. I'm sure it would have been an interesting comparison!

The datasheet (for my cells) suggests that 80 mOhm was the acceptable pass for initial testing. I do notice that when i conduct a 1A discharge on cells metered around 70-122 mOhm that the cells heat up (and in some are scolding hot!). A majority of cells sit around the 55-62mOhm range and they seem to be really good in both capacity and overall heat during charge and discharge.

So i guess you need to lookup the datasheet to get a real feel for what your cells can do.
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#9
intra the IR from Opus is not even close to being interesting. As said that depends on the pressure... So it basically depends on how you insert the cells into the tester. Therefore i would say ignore them and go with the SM8124 instead.

I have done a video showing how it can differ from 70 to 250 by just going back and forth or pressing.

As said IR is direct relation towards how the cell performs in terms of current vs heat. And yes for those that have time to do IR test do it. It goes fast to do a first check .
intra and 100kwh-hunter like this post
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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#10
(10-24-2019, 02:06 PM)daromer Wrote: intra the IR from Opus is not even close to being interesting. As said that depends on the pressure... So it basically depends on how you insert the cells into the tester. Therefore i would say ignore them and go with the SM8124 instead.

I have done a video showing how it can differ from 70 to 250 by just going back and forth or pressing.

As said IR is direct relation towards how the cell performs in terms of current vs heat. And yes for those that have time to do IR test do it. It goes fast to do a first check .

No one is using the IR from the Opus,  we were all using a YR1030 kelvin type (or similar model).
100kwh-hunter likes this post
Mobilis in Mobili
 
Cell count as of 10/10/2019
234 Cells >2000mAh, >80% Rem. Cap., 14 day resting voltage >4.12V
191 Cells of Everything Else
68 In progress
 
Aiming for 8 cells tested a day
More info on my Google Drive
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