For the bolded(by me) part:
Well, yes we do.
There is a general accepted principle of *square root of time* as the formula for calendar aging. I have seen some research reports that try to find other formulas for a more exact estimation. For us Tesladrivers, the square root of time is good enough. We in general have to little precise data anyway.
And some of the research about this implement a straight line despite the test data they have actually show a *square root of time* line
Example: This report try to imply the degradation actually is a straight line. Their real data stop before any straight line so their conclusions is not really valid in my eyes. ( source:
https://www.mdpi.com/2313-0105/7/2/28 )
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Here is actual test data, 18650 NCA cells. Do not remebmer if it says Panasonic but it fore sure is cylindrical cells with NCA-chemistry.
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660 days of tests(almost two years), we can see both the temperature dependent degradation as well as the SOC dependent degradation. (
3.45V = about 10-15% or so.
3.70 = about 45-50%
4.10 = about 90-95%
If I use the first clearly distinguable point on the 10C 3.7V graph and try to calculate the end point(660 days), I get 0.833% loss at day 76. using the squere root formula I end up at 2.45% degradation at 660 days and the real endpoint show about 2.2% degratation.
Of course a picture like this is har to use without fults.I did use pixel count to get as good values as possible.
For our use, the square root formula is fine. We have not the best data on our cells, as we most often have newer cells in the cars than the newest research report have. For example, the 2170L came on the market one year ago, and it probably takes one year for researches to get the hand on some of these cells and then it take another year to do a research and finalize the report.
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