I think the Honda SAE article is fascinating, thanks for that AnthonyW. They key new understanding for me is the fact that the electric motor (a voltage controlled device) is powered through a DC-DC inverter off both the battery and the generator. That is, it doesn't matter what the battery voltage is i.e. where it is on the discharge curve, the inverter can boost the voltage to what the motor needs to operate at the desired speed up to 100MPH. The battery chart they show there is KW (power being delivered to motor) vs KWH (power remaining in battery, which we can think of as a proxy for SOC). Because it's showing power delivered to the motor it's not directly related to battery voltage. That flat curve they show is to demonstrate how well they can extract useable power from a range of SOC.
Another way of looking at it is that using the rather less flat NMC voltage/SOC curves, the Honda system inverters compensates for voltage fall off and can deliver a flat looking voltage curve to the motor.
The low end of those curves are also interesting, Honda shows what they call Charge Depletion (which I would like to call EV driving) lasting until about 4 KWH remaining on the battery. That's about 24% SOC. That should be the point when the ICE turns on automatically, which is at about 2 bars on the displayed energy meter (10% if I recall). In real terms then, actual SOC (rather than what the meter or Hondalink displays) that Honda wants to operate the battery at ranges between ~20% to ~95% SOC. Mapping that to 0-100% on a meter means the thing is non-linear, but then we expected that looking at the battery voltage curves.
One final thing I want to say, is that battery chemistry is the "secret sauce" for EV manufacturers. I think Honda has developed a drivetrain/powerplant able to extract the most usable amount of power while preserving cycle times and battery life. Future improvements will be in battery chemistry. Which is why it was interesting to see Honda, this month, sign on to GM's battery development technology. To date, Honda is buying battery's and seemingly not engaged in battery design, unlike Tesla, GM and BMW to name a few. Partnering with GM will allow Honda access to some secret sauce and keep pace with developments.
