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Behaviour of cells in parallel.
#1
I'm starting this thread to start a discussion on how cells behave in parallel in a pack.
Before any of you poo poo this and say that all this has been studied and there are many papers out there on this I will concede and agree.
But of all the studies that I have read they all seem to use only the same cells from the same manufacturer whether Lipo, LiIon, LiFePO4 and are not mixing cells of different capacity, manufacturers and IR. Neither are they mixing low drain and high drain cells.
https://www.sciencedirect.com/science/ar...5316309995
https://www.sciencedirect.com/science/ar...5316300015
https://www.sciencedirect.com/science/ar...5315306911

The studies all come to a very similar conclusion that as cells degrade the interaction among them causes the cells to be stressed.
I am here to help us understand this phenomenon with down to earth layman's analysis and hopefully a good discussion.
I am no mathematician and most of the formulas the studies use are a complete mystery to me but I do understand charts and experiments.

I have built a board with an ESP32 Dev module and hooked up 4 INA219 I2C Bi-directional DC Current Power Supply Sensors to it. All the INA219
chips are from the same lot so deviation should be minimal if any at all. Along with the cell holder and the ESP32 with the INA219s attached 
there is a Buck CC/CV charger and a ZB206+ discharger on it. 
  

The data is going to an influx database to record  the 4 parameters that the INA219 produces.
Buss Voltage
Load Voltage
Milliamps
Shunt Voltage
The cells will be charged to 4.2V until mA cutoff saturation and then discharged to whatever voltage we want to 3.0V to 2.8 or whatever some of you want to experiment with. IR and voltage will be measured before the test and recorded.
We can experiment with a charge Voltage of 4.0V and a discharge to 3.2V etc. etc. etc. The range of parameters is unlimited.
You can participate in this study with suggestions of what cells to test in combination of each other.
All of this data is then graphically represented on a grafana dashboard for analysis.
I have added annotations and will continue to do so at the start of each cycle on the milliamp chart to indicate the cell part number IR and Voltage.
As this data accumulates you are more than welcome to look at it at any time http://wolftech.mynetgear.com:4562/d/gsj...5786145474 username guest password guestw
To view the data correctly choose a time frame 

Once you learn how to use grafana you can drill down to the time frame you want to see and then can extrapolate your own conclusions. Anything prior to 8/11 is experimental and should be disregarded.

The first set of cells you see in this graph are

Discharge curve at 2 Amps for the pack and charge curve at 4 Amps for the pack

The second set are all UR18650FMs which are undergoing a 1A per pack discharge right now. Combined mAh of the 4 is 9615mAh so it will take at least 9 hrs do discharge at that rate. They have been discharging for almost 4hrs.


Questions, comments, rants, and raves bring them on. Tongue

Wolf
thunderheart, completelycharged, chuckp And 1 others like this post
If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 8/7/2019
Total Number of Cells Recorded and processed                 6149
Total Cells required for PowIRwall                                   2856
Total Cells ≥2200mAh, ≥80%, ≥35mΩ, ≤75mΩ, ≥4.12V   2760
For Info Google Drive
Not your average Wolf       
            Cool
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#2
This is a Cool experiment. Always wondered with varying Cell health the interaction between the cells.
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#3
Brilliant. Data !!!!!

Suggestions
1. Same cell make/year but more than one cell where the capacities are different, i.e. a cell with 2000mAh another with 1900mAh and a third 1800mAh. Reason : if the accuraacy of your readings are good it may well show up some other traces to actually measure the remaining cell life..
2. Measure changes in IR with different discharge / charge currents, through the voltage range, due to internal chemical reaction differences/distributions. Again this may end up showing a cell life correlation

Loads more suggestions (like over charge and undercharge voltage profile and IR)...

Brilliant.

Separate thought... you need the data in excel/open office calc in order to actually play with the numbers and see the real gems... The charts are great, but so it a 6ft target from 100m away, it's the small subtle differenec and detail which may actually prove to be a real will that everyone has seen but nobody has noticed.
chuckp likes this post
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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#4
(08-14-2019, 03:47 PM)completelycharged Wrote: Brilliant. Data !!!!!


Separate thought... you need the data in excel/open office calc in order to actually play with the numbers and see the real gems... The charts are great, but so it a 6ft target from 100m away, it's the small subtle differenec and detail which may actually prove to be a real will that everyone has seen but nobody has noticed.
First off thanks

Ah yes the close up view.
Well I can view the chart by the minute if need be. And so can you!

Or here is a 5 minute x axis view of a different time section.


Alternatively the data can be exported from influx to a csv file.

Better yet as a guest you can also export the data once you select a time frame to CVS format.


Select "Series as columns"

Import into Excel or any other sheet program


And Bobs your uncle. Now you can view, manipulate, graph, and lord knows what else you want to do with the raw data to your hearts content.

Update:
I have created another dashboard with just the raw data that you can download on each individual cell and a combined chart of all the cells.
http://wolftech.mynetgear.com:4562/d/JIl...24h&to=now
same username  guest and password guestw
 
And the export of the All Cells Raw data is perfect. So have fun.
 



Wolf
Nemo, chuckp, completelycharged like this post
If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 8/7/2019
Total Number of Cells Recorded and processed                 6149
Total Cells required for PowIRwall                                   2856
Total Cells ≥2200mAh, ≥80%, ≥35mΩ, ≤75mΩ, ≥4.12V   2760
For Info Google Drive
Not your average Wolf       
            Cool
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#5
Very Nice dashboard.  I'm going to have to look into this for my setup
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#6
The real interesting calculations will be the trace of difference between the cell voltage/IR for a given charge/discharge Wh for different cell ages.

If you have 4 identical cells (different mAh capacities) I would be really interested playing with the data to see where it goes.....

I think there may be a way to determine cell life and the data you are hopefully able to generate could prove this.
Not sure if you read my earlier thread that had no responses, lol.
https://secondlifestorage.com/t-Charge-a...-data-help
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 ......
Reply
#7
(08-14-2019, 10:53 PM)completelycharged Wrote: The real interesting calculations will be the trace of difference between the cell voltage/IR for a given charge/discharge Wh for different cell ages.

If you have 4 identical cells (different mAh capacities) I would be really interested playing with the data to see where it goes.....

I think there may be a way to determine cell life and the data you are hopefully able to generate could prove this.
Not sure if you read my earlier thread that had no responses, lol.
https://secondlifestorage.com/t-Charge-a...-data-help
Completlycharged,

Just read your post from 06-17-2018, 09:03 PM 
https://secondlifestorage.com/t-Charge-a...-data-help
Well that was posted just a little before before my time (I joined  09-25-2018, 07:27 AM) but somehow the concept of this study has struck the same cord in both of our minds.

Quote:Ideally I would need 1 second measurement data of voltage and current and probably a couple of PDh students looking for a thesis... so anyone interested in helping get in touch, pm or post.


Well I did put a 2 second delay at the end of the loop to give the database a breather from all the data thrown at it, but I can probably remove that. I do have a 960GB SSD drive I will be installing on this laptop soon and that should improve writability a bit.
Phd well sorry you are stuck with me.  Tongue

I will break apart some packs and try to get a set of identical cells with reasonable IR that I know will pass ,be marginal, and fail.
As you know I have a pretty good handle on IR and know how to judge a cell by it.
So I will try to get 4 cells with different IR readings out of the same pack hence the cells should be from the same lot.
I will capacity test them first on my SKYRC MC3000 and leave them at a state of discharge. Then charge them on my (Phd Big Grin ) board at 
4A (1A each) and discharge at 2A (500mA) each. Of course recording V ,mAh. and IR throughout all the steps.
Lets see what happens. I should be able to get to another pack break open session this weekend.

Let me know if that is what you are looking for and anything else you can think of.

Wolf
completelycharged likes this post
If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 8/7/2019
Total Number of Cells Recorded and processed                 6149
Total Cells required for PowIRwall                                   2856
Total Cells ≥2200mAh, ≥80%, ≥35mΩ, ≤75mΩ, ≥4.12V   2760
For Info Google Drive
Not your average Wolf       
            Cool
Reply
#8
This is very interesting Wolf!
I am looking forward to the results.
I think it may shed some light on how to select or reject cells to use and/or build good, usable packs.
Korishan likes this post
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#9
You can easily push data into inlfux every 10ms if you want... With normal drive... Influx is a time series database.... Just make sure you set it up correctly in terms of resolution and cache in front so it doesnt write each value but instead for instance 5sec at a time.

Doing this you can like example the raspi project i use... I write 500 values per second into that Wink On SD card.

Im not going to comment the work right yet.... Its a well layed out work for sure and I think what you will find is "suprising" Smile
Im very happy you do it so people get to see it. I was thinking of it but life catched up with me unfortunately 2 years back and never got to show my findings.

Keep it up guys! VERY well done so far.
chuckp, Oz18650, Korishan like this post
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#10
(08-15-2019, 05:39 AM)daromer Wrote: You can easily push data into inlfux every 10ms if you want... With normal drive... Influx is a time series database.... Just make sure you set it up correctly in terms of resolution and cache in front so it doesnt write each value but instead for instance 5sec at a time.
Thanks daromer.
I'm looking at the influxdb.conf file and there are a bunch of variables that can be set for the database. I am pretty much running it in default mode right now. Let me know what settings you would change. I assume they are in [data]  and the TSM engine. If I run my query at 1s I get data at about every 3 to 5 seconds which isn't that bad. I do get a bunch of nulls though.

I will eliminate the 2s delay in the sketch at the end of the loop and see what happens.

I have downloaded the raspi image from your website but haven't played with it yet. Don't have a pi yet been concentrating on ESP8266 and ESP32.

Wolf
If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 8/7/2019
Total Number of Cells Recorded and processed                 6149
Total Cells required for PowIRwall                                   2856
Total Cells ≥2200mAh, ≥80%, ≥35mΩ, ≤75mΩ, ≥4.12V   2760
For Info Google Drive
Not your average Wolf       
            Cool
Reply


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