# Battery Characterization System for 18650 Cells



## Karter2 (Nov 17, 2011)

Could you elaborate a little on exactly what tests the unit does , and how long the test cycle is.
The description is a little light on detail !


> ?..... The system cycles cells, runs cycle tests to determine capacity, sinusoidal and pulse tests to characterize impedance, and can estimate state-of-health when compared with benchmark values for a particular cell model.The user experience will be a process of setting the batteries in the device, starting the program, selecting a test to run, and swapping the cells a few hours later .....


Suggestion...
You might want to consider ensuring the unit can handle the newer 20650, 20700, and 21700 ~5Ah cells, that are being intoduced for power tools and Ebikes.


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## dcambron (Dec 11, 2016)

Gladly!

The test unit itself is capable of injecting charge and discharge waveforms at up to 2.5A (And we're experimenting with thermal management to get this higher), and measuring current, voltage, and temperature with reasonable precision for the price point. It will have several different charge and discharge modes you can command it to use, such as a constant current cycle, sinusoidal charge cycle, and pulsed cycle, and nearly all of the parameters such as pulse duration, frequency, current magnitude, voltage setpoints, etc, are user-configurable. There will also be a mode that does a constant current charge/discharge cycle with occasional interruptions to take an impedance measurement using a sinusoidal sweep or a resistance measurement using a DC pulse.

All of that data is streamed over a USB connection to a PC running our software. The software is there to guide you through the entire process of building a battery pack. You'll be able to go through a wizard that will ask you how many batteries you have, and will direct you to label them. It will then schedule a bunch of tests to be run on the cells (with specifics depending on the settings you choose), and will direct you to place certain cells in certain slots on the tester. It will then start the tests and show progress on GUI.

Running a standard set of tests (a charge cycle to top off the cell, rest, full discharge cycle with impedance measurement, rest, full charge cycle with impedance measurement) will take about 3 hours depending on the cells and settings.

At that point, you'll be directed to put the next set of cells in the tester, and this process will repeat until the queue of tests is empty. Keep in mind that you can daisy-chain multiple units so you can do many cells at a time. We've been in contact with some people trying to build DIY Tesla Powerwalls--and they're going to have hundreds of cells to test! If you have 4 Batlabs you can do 20 cells at a time, etc..

The real magic happens after all of the tests are run and all of the data is collected. We're working on an algorithm that will optimally match cells together in a battery pack given constraints like the number of cells you have, the max number of cells you want to use in the pack, the min/max system voltage, etc. So at the end of all of your tests you will get a printout of which cells you need to combine in parallel to make modules, and which models you need to combine in series to make a pack.

As for your suggestion about other form factors, you bring up a good point. There has definitely been an uptick in interest lately in those other sizes. We decided for this initial run of units to be designed specifically for 18650 cells, but told ourselves that it wouldn't be difficult to do other production runs of the product with different form factors if there was interest....we're working on making a design that could fit different sized cell holders on the same PCB so it would just be a matter of swapping to the right cell holder for your cells.

Anyway, feel free to ask more questions or give more suggestions! My goal is to make a product that could actually help people in the eV/eBike community make better/safer battery packs, so we welcome any suggestion that will help us get there!

Regards,
Daniel


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## Karter2 (Nov 17, 2011)

OK, thanks for the update.
..a few comments,.
I understand this is not intended to be a "full function" cell tester, but more like a cell comparator/assesment system...
....but i do feel it is important to be able to test discharge at least at 1C rate if not 2C or 3C for the higher power cells.
That ,considered together with the capacity of 3.5 - 3.6 Ah , (or higher for the new format cells) then i hope you can increase that current handling level.
Presumeably, with the configureable discharge parameters it will be possible to establish the AC and DC internal resistance at various states of charge ?
In addition to the preconfigured output of data collected (print out ?) , will there be facility to log all data files for storage and analysis in 3rd party software ( eg Logview etc)

Thanks again, and success with the project


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## GoElectric (Nov 15, 2015)

Hi. Useful device, thanks for doing it. 

I'm curious as to the impedance characteristics for 18650's - do you have any graphs vs temp or SOC? I've not been able to discern a pattern while testing mine, which came from a Tesla, but surely there has to be some change. 

Thanks and good luck. I know a couple of people who might be interested.

Jim


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## dcambron (Dec 11, 2016)

I'll take a stab at a few more of your questions:



> but i do feel it is important to be able to test discharge at least at 1C rate if not 2C or 3C for the higher power cells.
> That ,considered together with the capacity of 3.5 - 3.6 Ah , (or higher for the new format cells) then i hope you can increase that current handling level.
> Presumeably, with the configureable discharge parameters it will be possible to establish the AC and DC internal resistance at various states of charge ?
> In addition to the preconfigured output of data collected (print out ?) , will there be facility to log all data files for storage and analysis in 3rd party software ( eg Logview etc)


You bring up valid questions, and we are working to address these and also balance thermal management and cost. Something we're looking at is that even with a 1C continuous current rating, we could put in provisions for DC resistance to be measured using a higher magnitude (2C) pulse for a few seconds, and the momentary spike in current would not be enough to cause a heating problem...keep in mind that the thermal limit we're currently seeing is actually the case getting too hot, the components are staying within their ratings. On a related note, we've actually had a backer bring up the suggestion of using multiple channels in parallel so that instead of testing 4 cells at 1C, you could test 2 cells at 2C, for instance. If anything, it is an interesting thought that we will look into.

You will certainly be able to collect impedance data over SOC and export the data. This was actually one of the first requirements I gave to the software designers on our team. As for the data output (I called it a "print-out" but what I really mean is a text file), everything will be stored in text files in standard formats that will be easy to post-process. Right now, we've designed out system to have 3 'levels' of data files that are produced. Raw data containing voltage, temperature, etc is level '1'. Level 2 is the impedance and capacity data that is generated from level 1 data, and will be in separate files. Level 3 data is derived from level 2 along with additional constraints, and will be the data suggesting cell matchings and pack configurations. Each level of data will be stored in easy-to-process text files.




> I'm curious as to the impedance characteristics for 18650's - do you have any graphs vs temp or SOC? I've not been able to discern a pattern while testing mine, which came from a Tesla, but surely there has to be some change.


As a matter of fact, I can answer this question to some degree for LiFePO4 cells. While doing MS thesis research  on current estimation, I saw some moderate temperature dependence and also SOC dependence for the internal resistance of the cells I studied. You can find some relevant info/graphs on pg 30-32. I would presume to see a similar trend in 3.7V chemistries....but data like this was actually part of the motivation for the 18650 battery tester project. I wanted a way to get better data in a more 'standardized' way so I could more easily compare one cell to another.

Thanks for the interest! I'll be happy to answer any more questions or concerns.

Regards,
Daniel


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## GoElectric (Nov 15, 2015)

Hahahhah. Eigenvalues. For anyone who doesn`t have a clue, I think that means they are the numbers which fit a whole bunch of data - ie: resistance vs load etc.... It looks like you are saying the greatest dependancy is on SOC - increasing? Then load current - decreasing?

From what I read of it, I thought your thesis was pretty well-written. Totally fine to use that kind of jargon (eigenvalues) of-course!


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## dcambron (Dec 11, 2016)

GoElectric said:


> Hahahhah. Eigenvalues. For anyone who doesn`t have a clue, I think that means they are the numbers which fit a whole bunch of data - ie: resistance vs load etc.... It looks like you are saying the greatest dependancy is on SOC - increasing? Then load current - decreasing?
> 
> From what I read of it, I thought your thesis was pretty well-written. Totally fine to use that kind of jargon (eigenvalues) of-course!


haha, yeah I actually meant pg 30-32 in the text, which is absolute page 43-44.

Equation 35 shows that cell resistance depends on temperature, and figure 4.6 shows it depends on state of charge.

As for the Eigenvalues on pg 20-23 in the text, what that's actually saying is something to the effect of "It takes a long time for the estimate to converge on the actual measurement, but it will eventually converge". The big idea was that you can make a good guess what the current flow is in a battery pack (without measuring current directly!) by accurately measuring the voltage of the pack over time and knowing how the internal resistance behaves. So yeah, there's a bit of jargon in there. All part of the fun.


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## GoElectric (Nov 15, 2015)

Thanks. Good work and good luck with your product.


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