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Achieving valid self-discharge measurements: How external factors impact results

2021-01-08  |  5 min read 

I was set to present a topic in person at the Novi Battery Conference in September, in Novi, Michigan. Things were looking good up until mid-August, and then the live event was finally cancelled due to the pandemic. However, the show must go on! The organizers opted to do a virtual event, which took place this past November. As the number of presentations were considerably reduced, I count myself as being one of the lucky ones who they decided to retain for the virtual event. The title of my presentation was “Achieving valid self-discharge measurements: How external factors impact results.” This was a nice sequel to what I presented at the Novi Conference in 2019 and wrote a blog post on; “Shortening Lithium Ion Cell Manufacturing Time: A Comparative Study of Two Methods of Making Self-Discharge Measurements.” (Click on the title to learn more!)

For a little background, lithium Ion cells gradually discharge even when they are not connected to anything. Some self-discharge is normal. However, excess self-discharge indicates potentially catastrophic problems within the cell. Due to this, all cells are screened in manufacturing for self-discharge. There are two main methods for testing self-discharge; the delta open circuit voltage (OCV) measurement method and the potentiostatic method for measuring self-discharge current. These are illustrated in Figure 1.

Figure 1: Self-discharge measurement methods

Very briefly, for delta OCV method, the Li-Ion cell’s drop in OCV is measured over an extended period, typically days to weeks. A drop in OCV is an indirect indicator of loss of charge. In comparison, the potentiostatic method directly measures a cell’s internal self-discharge current, typically on the order of an hour. This is accomplished by holding the cell at a constant potential with a very stable external voltage source. At equilibrium, the current being supplied by the external source equals the cell’s internal self-discharge current. I have previously written about these two measurement methods in much greater detail. For reference, “Keysight Solutions for Measuring Self-Discharge of Lithium Ion Cells Achieves Revolutionary Reduction in Test Time” (click on title to access) is a worthwhile post to review to learn more about cell self-discharge and its test methods.

So now you can understand why it is a top priority for testing cells for self-discharge, and why it is vital to get consistent and valid results. This may seem straight-forward in principle. However, in practice, most find it very difficult. The underlying challenge is that self-discharge is far from being relatively fixed and constant. It is greatly impacted by several external factors, which is what my presentation for the Novi Battery Conference was about. Top external factors affecting self-discharge measurement are illustrated in Figure 2. Note that some factors impact just the cells, just the measurement methodology, or both. This is true regardless of which measurement methodology is used.

Figure 2: External factors impacting cell self-discharge measurements

While the virtual Novi Battery conference event for 2020 has come and gone, there is now another opportunity to learn about what I had presented. I turned my presentation into a paper, which has recently been published on-line, available for your review. So, if you want to learn more about how these external factors impact self-discharge measurements, click on the following title: “Measuring Lithium-Ion Cell Self-Discharge” to access. I believe you will find it insightful and educational, just as I did when I conducted all the testing to collect the results incorporated into this paper.