What exactly is the model then? It's essentially an approximate SOC point to kWh mapping. The mapping itself is speculative, but it seems to be be in alignment with other observations voiced on this forum and with the data gleaned from the few and far between comments from Nissan. I also found some very helpful information on the Tesla owners forum, and I believe that the Leaf is much closer to the Roadster in terms of charging protocols and battery care than we think or Nissan has admitted.
But back to the model. SOC 281 is not valued at 100%, because it very likely does not represent 100% pack capacity. It's the full available charge, which is less than 24kWh for the sake of battery longevity. The mapping I'm proposing assumes 320 SOC points representing 24kWh. Each point would be 0.075kWh or 75Wh. There is a silent reserve on both ends. It's likely 20 points below zero, that's what's left in the pack when the Leaf stops dead after turtle. There are another 19 points above 281 to make it full 320 SOC points. I took this metric and added pack SOC % and kWh estimates to your chart.
My comments are based on my knowledge of laptop battery performance/operation, and my background as an electrical engineer working with microcontrollers, sometimes within laptops. (Consider researching how laptop battery controllers estimate remaining charge, i.e. "gas gauge" functions.)
The 0-(320) SOC Points suggest to me a working measure of the energy remaining in the battery pack based only
on Coulomb counting (or integrating battery current over time).
There are factors that affect battery capacity that have nothing to do with battery current.
Battery temperature: 1) discharge efficiency, 2) self discharge rate, and 3) actual capacity.
Discharge current: 4) discharge efficiency (faster discharges are more expensive in terms of actual capacity).
Time since last charge: 5) actual capacity (due to self discharge).
Age of battery: 6) discharge efficiency, and 7) actual capacity (age reduces capacity - even batteries sitting on the shelf doing nothing).
Cell equalization: 8) each cell in the battery pack will respond differently to each of these effects.
Battery use history: 9) number and depth of discharge/recharge cycles.
At best, a BMS can only estimate the affect of each of these factors, and assign what the current 100% SOC is in terms of SOC Points.
Because this is only an estimate, the actual (and current) 100% SOC needs to be reset (recalibrated) by periodically performing an equalization charge to fully charge all battery cells to 100%. As soon as all cells are charged to 100%, the BMS resets the SOC to 100% and SOC Points to (320?). After that the 100% SOC Point only reduces until the next equalization.
Each of these effects only reduce the SOC Point that is considered 100% SOC. Performing an equalization charge tends to restore the 100% SOC Point, but does not exceed the previous 100% SOC point (hence battery aging and capacity reduction).
Assuming the operator regularly charges to 80%, the 100% SOC Point would slowly decrease over time and discharge cycles. Performing a 100% charge would tend to restore the 100% SOC Point back to its previous level. Has anyone captured data that looks like it might be an SOC Point that slowly decreases with each discharge cycle between 100% charges?
Dividing the current SOC Point by the 100% SOC Point should give a very good idea what the BMS thinks is the remaining %SOC.
1st Batt Bar @ 8.5k mi. 29Aug2012 ..... 2nd @18.2k mi. 14Jun2013 ..... 3rd @(21k) mi. Aug2013 ..... 4th @ 34.0k Oct 5, 2014
SOH= 63, Hx=41.74 AHr=41.36 ..... Replaced ..... SOH=100, Hx=99.98 AHr=66.14 ..... Blue 2012 SL, 3 Yr Lease ends Dec2014