Tag: Testing

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Individual Cell Testing in Large Battery Packs

I have been very fortunate to be part of the Midnight Sun Solar Car Design Team at the University of Waterloo (uwmidsun.com). As the Battery Lead for our next solar car (MSXIV), I am responsible for designing, prototyping, testing, manufacturing, and integrating the battery.
This discussion/post/article/whatever you want to call it will go through some of the considerations when building a large battery pack for high performance applications including electric vehicles. This is not as critical for the performance of say a home-built powerwall from recycled cells, but from a safety perspective is probably more important in such an application as the cells may already be degraded (in invisible ways – pending internal shorts) from their first use.
Lets start off by posting a PDF of some research/literature review that I did a few months back to determine if individual cells should be tested for a solar car battery pack. The result of my evaluation is that basic individual cell testing should be conducted, but based on time frames, a full capacity measurement will not be done. If we had access to a highly parallel capacity testing rack (which I have had some ideas on building one – might come in the future) then capacity testing would be good to do.
Click the link below to download the PDF.
The testing was done with a Keysight B2902A SMU and a custom scale made with Phidgets hardware. Our goal was to be able to detect manufacturer defects that could cause cells to heat up or fail prematurely. To this end, we testing for DC and AC Internal Resistance, Self Discharge Current, and cell weight, and an estimate of capacity through differential capacity through the capacity ration.
A custom scrip tin Python was created to interface with both instruments and automatically collect the data into CSV files. The testing was completed in a short timeline (36 hours) and, we collected data for 1400 cells. The CSV files collected made up over 6GB of data.
To process the data, more python was used – if you can’t tell yet, I really like python, especially when tools like matplotlib and numpy make data processing super easy.
These will be following the guide I have outlined here:
Once we have processed the data, it will be used to identify outlier cells and match them in order to create and most balanced pack as outlined in the Individual Cell Testing Evaluation Guide linked previously.
With all of this data, we hope to create a battery pack that will power Midnight Sun XIV on its 3000km journey across the United States during the American Solar Challenge in the summer of 2020.
Written By: Micah Black
Project By: Micah Black
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LiitoKalaa Engineer Lii500 Reliability, Accuracy, and Repeatability Test

Accurately testing 18650 li-ion cells is a very important step in the process of re-using cells from laptop battery packs, or designing a new battery pack in order to create matched modules and maintain a minimum amount of balancing current to keep the modules balanced.
During my first term at the University of Waterloo, I joined Midnight Sun, the solar car student design team and am working on designing a new battery pack for the next car, MSXIV (Midnight Sun 14). During this process, we are looking to accurately test every single cells that we putting in to the car in order to determine their capacity and internal resistance. This will allow us to create perfectly matched packs if the testing is completed accurately.
New cells are being used in this car, and the tests must be able to distinguish cells that fall within the manufacturer’s specified tolerance ranges for the cells. That means that the testing method that is chosen must be accurate to 10mAh for the capacity and ideally less than 1mOhm for the internal resistance.
And so started my journey of figuring out how to test all the cells in a timely manner while not spending tens of thousands of dollars on proper commercial testing equipment.
Starting with, one of the cheapest and most popular cell testers on the market, the LiitoKalaa Engineer Lii500.
I have 9 such testers, part of my cell testing station, and used 9 different cells to test each one, one slot at a time.
Each module was tested with one cell in the same slot multiple times to determine the repeatability of a measurement in the same slot, then the cell was moved to a different slot to see how the measurements compared.
The results can be seen on this spreadsheet, and were somewhat surprising. For tests of the same cell in the same slot, the values did not vary too much, within a range of 20mAh. However, when the cell was moved to test different slots, results were changed to a spread of almost 100mAh for some testers, with the average spread between the 4 slots around 50mAh.
Given these results, these testers are unsuitable for determining small differences in capacity between a batch of new cells, but for testing cells from laptop pulls they are perfectly acceptable. Keep in mind though, that the results will be within a range of around 50mAh from the value that they show.

Test Conducted By: Micah Black
Written By: Micah Black