Using the PHEV Watchdog I recorded some voltage measurements for a range of reported SOC values. The measurements were taken after an at least 3h rest after charging or driving, so that the battery can rest and the BMU has time to reevaluate the SOC. The range goes from 20% SOC (the lowest I was able to get) up to full charge. As you see in the attached plot, in the studied range the points are nicely fitted with a linear function (blue line). Moreover, SOC at 4.1V is 31.1Ah, which is exactly what the dog reports as my battery capacity (77.8%).
The red line has the same slope, but is shifted so that capacity at 4.1V is 40Ah instead of 31.1Ah. The point of this: IMO if we could add data obtained from two more cars (1) a new car, and (2) a car after the manual Lindqvist capacity reset, we could figure out how well the BMU is doing in estimating the true battery capacity. This could also provide us with means to build an independent battery capacity model.
If I would subject my car to the Lindqvist reset, the new measurements would plot on the shifted red line in the figure, give or take the collapse of the linear fit in the left side of the plot for low SOC (the SOC-Voltage relationship is no longer linear for low SOC). This would essentially mean that the reset simply causes a deeper discharge of the lower buffer. If we had measurements for such a car, and we could actually observe the non-linear regime, it would give us insight into the true SOC, and consequently true SOH.
On the other hand, a new car should show a different fit. Assuming that the BMU attempts to keep the voltage at the bottom of the discharge the same for all cars (~3.8V) and that obviously the same voltage is put as the roof when charging (4.1V), new cars should plot roughly on the black dashed line in the second figure. Another option would be that the BMU tries to keep a constant charge as the lower buffer (here around 6.3Ah). This would mean that the voltage at the bottom would be have to vary. Again, having such data for a new car would tell us what it is that the BMU is doing.
Would anyone be interested to contribute some data points? Before you record the data, please wait at least 3h after charging or driving.