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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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