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DanJohnson

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Aug 24, 2021
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2
Hello, I'm a first semester graduate student in the USA and I have recently gotten super interested in Lithium Batteries! I have a ton of used 18650 cell's I've harvested from all sorts of products and while I was building an E-bike battery last year I was beating my head against the wall trying to test close to 200 cells using an OPUS BTC3100 and waiting hours to test 4 cells at a time. This gave me the idea to dedicate my research to finding a way to create a faster way to test used 18650 cells quickly and accurately without having 20 testers on a giant power supply and still spending hours at a time.
I was originally going to try and develop an internal resistance tester that uses electrochemical impedance spectroscopy (EIS) to instantly determine Internal resistance and save you the time of running charge discharge tests, but then I realized that was going to require a lot of expensive equipment that I most likely wouldn't be able to afford with my $1500 budget. I also just found this site and due to the amount of very knowledgeable and experienced professionals and hobbyists on here I figured y'all might have some ideas.
My goal is to find ways to make it easier for people to quicker way other than charge/discharge capacitance tests to accurately determine if a used 18650 cell is worth keeping or recycling. I see a huge amount of waste from used 18650 cells that still have a long usable second life, but due to difficulty in testing they are thrown out or collected by "recycling" companies but ultimately ending up in the landfill.
I would love to hear any input from this community about possible technologies to investigate, ideas for tools that would make testing used cells easier, gaps in the community knowledge that could benefit from formal research or really any suggestions.

I look forward to hearing from you all and I'm excited to have finally found the community I've been looking for!

Best,

Dan
 
Joined
Mar 7, 2018
Messages
1,054
Try this

I don't think you need expensive kit, check Wolfs threads of cell testing, experiences and how he has tested cells with a smart approach.

For short duration minimal test time, highly accurate kit, however, extend the time and you compensate for the measurement accuracy.

For cycle degredation high accuracy energy in/out is needed if you only want to measure with a few cycles. Time is what you have and just parallel up.
 

OffGridInTheCity

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Dec 15, 2018
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1,738
Battery University has been out there for a long time... but I actually based some of my key design decision on the long life page. If you haven't see it you might check it out for some tid-bits - https://batteryuniversity.com/ and in particular I've used this one for my operating guidance - https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries

Today I'm an 1,068 daily cycles at avg 40% DOD on my oldest 18650 battery in my DIY powerwall with no detectable loss of capability yet.
 

italianuser

Member
Joined
Feb 25, 2020
Messages
359
Welcome to you researcher! :giggle:

Well, testing cell capacity via C-D-C cycle is a very practical way of finding actual cell capacity.

I'm pretty sure that a high-technology density/chemistry analysis of a cell is somehow possible, but that's surely not available at low cost.

For what concerns your goal of obtaining an accurate measurement I can tell you this: I've just finished measuring 1046 cells for a total of 7934KWh, using three Liitokala 500 chargers, testing about 36 cells/day. More than once I really desired having ten chargers which would bring my numbers up to 120 cells/day which would be quite fast.

Consider that talking about second hand cells it's advisable to go through a 10/15 day test for each cell, following a strict procedure that will help getting a quality set of cells. Quite a good protocol is described in this thread: https://secondlifestorage.com/index.php?threads/18650-harvesting-flow-charts.9714/#post-66506.

Getting a really accurate measurement is not possible unless your testing procedure is aware of both cell datasheet and load you will connect to the cells.

For example, look at datasheet for Panasonic/NCR CGR-PD cells:

Panasonic PD discharge curve.jpg

For a new cell, testing at a 0.55A discharge rate would give you a measurement of nearly 3000mAh. While testing this same cell at a 5.5A discharge rate would give you a measurement of about 2750mAh.

This is Samsung 22F datasheet:

Samsung 22F discharge.jpg

Again, testing with Liitokala 500 the measurement with discharge rate at 0.5Ah will be higher than an Opus with discharge rate set a 1.0Ah.

Now, as you surely know, when we mass test cells we ignore cell model and set the discharge rate at a fixed 1000mAh (or 500mAh rate on Liitokalas, which is the reason that Liitokala gives a higher capacity reading -as @Wolf teaches us). So when doing mass test we obtain a reasonable capacity measurement, but in real usage of that cell it's possible we can get a higher capacity out of it (or lower, depend on the load).

So, for once here in the forum we can say that Liitokala 500 measurements for an application where load is 0.5A are more precise than the numbers you get with any other faster charger set at 1.0Ah discharge rate.

Going to a faster testing method: using a higher charge/discharge rate means that each test is faster. So if you make your DIY charger which can push more amperes into cells then the test will be faster (keeping in mind the datasheet maximum rates). But, it may not make sense testing cells at their maximum ratings when your load is maybe as little as 0.5A.

[EDIT: ops sorry Korishan! I made a normal thread post out of an intro]
 

OffGridInTheCity

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Dec 15, 2018
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1,738
In which Voltage Range, do you keep them?
4.0v/cell high and 3.54v/cell low are the overall settings.
In Winter its in the lower portion of the operating range - 3.8v hi (on average) - 3.54v low - due to lack of sun to get it up to 4.0v.
In Spring/Summer/Fall it will more often hit 4.0v/cell hi.
The overall yearly average is 3.85v high to 3.54v low.

The overall battery bank was gradually enlarged to achieve these ranges w/respect to the home consumption patterns. I wish there was more data - for example would it be worth it to auto-adjust the range a bit higher in winter for longer life? or would it pay to double the battery bank size? Just not enough data and batteries are a bit 'variable' in their nature.
 
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Wolf

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Sep 25, 2018
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My goal is to find ways to make it easier for people to quicker way other than charge/discharge capacitance tests to accurately determine if a used 18650 cell is worth keeping or recycling
Well that was my goal also. That is why I developed the IR cheat sheet. https://docs.google.com/spreadsheet...ouid=105132588382800520118&rtpof=true&sd=true

For a reasonably accurate IR reading at that low of an ohmage it needs to be accomplished with the 4 wire kelvin methode. Also most manufacturers post the "acceptable" mΩ of a cells impedance at 1khz AC. This measurement can be accomplished with quite a few mΩ measuring devices.
I personally prefer the RC3563.
My workflow basically is liberate the cell, check Voltage and IR, as long as IR is acceptable for the specific cell part number and manufacturer, and the Voltage is reasonable the cell has a very good chance of being good.
I have got the procedure down to a science and pretty much can tell you if a cell will pass the muster or not by examining the date code, cell chemistry (ICR, INR, or Hybrid), IR and V before it even gets close to a charger/tester/analyzer for a C/D/C check.
My sheet of over 6000 cells tested with IR and mAh results has a lot of info that can be gleaned from it.


Also check out this thread.


Have fun
Wolf
 
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DanJohnson

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Joined
Aug 24, 2021
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2
Thank you everyone who contributed! I learned a lot from the links and resources you all connected me to. Wolf, big thanks to you too, your data excel sheet has been extremely valuable in helping me design my idea and I'm working on a prototype for could potentially increase the speed of your testing method that I'll share with you all when I'm done! Thanks again, you all are awesome!
 

Oberfail

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Joined
Jun 22, 2021
Messages
57
If possible to implement, measure the IR + Voltage before testing, even if its under 2V or so and once more after its charged to 4.20v at the end.
The data could be very usefull for future reference, as i know someone who is working on a tool to guesstimate the reliability of a cell at different recovery voltages which already works well, just needs more data to be feed.
 
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