Today I had my car to the workshop for the steering recall (which I think most of the world did last winter
).
Took the opportunity and asked for another battery report. The SOH had changed from 99.2% to 99.5%. So I checked the deterioration numbers from these two occations and think I may have found a feasible formula for the SOH computation!
Here's the data:
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Date SOH MaxDet MinDet
27/9 99.2% 11.5% 10.0%
3/11 99.5% 11.2% 9.7%We know that the SOH stays at 100% quite long, and the deterioration values are changing frequently when triggered by the right charging event. So I made the assumption, that if the SOH is computed from the deterioration numbers, it must actually start a bit above 100% for a brand new car. The instrument could display 100% though.
To get my two datasets fit in a common formula, I came up with this:
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SOH% = 110% - Average(deterioration) = 110% - (MaxDet+MinDet)/2If the SOH is calculated like this, it also means that in a situation of battery warranty at 70% SOH, the average deterioration would be 40%..
Side note:
The 110% baseline would correspond to a total energy capacity of 27kWh * 1.1 = 29.7kWh, or Amp capacity of 75Ah * 1.1 = 82.5Ah. This is just below what has been measured by the AVT test lab. Actually I think the cells were slightly over-speced by SKI for the test (4.3V max and 2.5V min), probably using the ABSOLUTE maximum and minimum voltages. I'm quite sure that SKI would never allow these extremes in any situation. For example, when I ran the battery flat this summer (on purpose), the HV battery was at 297V, which corresponds to 3.09V/cell.Again, all this is still very speculative because of lack of data, but I think it makes sense as a starter point. The SOH formula should be easy to implement in Torque Pro too. And more data needed.
