In some recent posts, we were able to find out and discuss some important details about how the Volt's li-ion battery pack is intended to function. One of the most critical facts we obtained from GM is that the 40 mile all-electric driving range will occur within 50% of the batteries maximum charge, or 8 kWh out of 16 kWh total. This translates to 200Wh/mile of energy consumption.

We also looked at the ability of the combustion engine (53 kW maximum) to keep the battery charged and how it might operate to do so. This generated some very intriguing discussion and further questions.

I went back to GM and had some discussion with other sources familiar with the Volt's engineering, and have been able to elucidate the following more accurate facts:

As per Rob Peterson, GM spokesman, the battery will operate in the 50% "swing" zone, but actually, the batteries full point will be 80% (not 100%). So its charge state will actually vary between 30% SOC and 80% SOC. This translates to the following analysis of battery capacity:
theoretical capacity         100%                16.0 kWh (not realized under normal conditions)
highest recharged level     80%                 12.8 kWh (after being plugged in)

charge sustaining level     30%                  4.8 kWh (after ~40 miles of driving)
empty                             0%                    0.0 kWh (not realized under normal conditions)
In terms of the on-board generator, the peak power of 53 kW will rarely be used, only in extreme conditions. Peak efficiency will be at around 30 kW, which is what the car should require at 65 mph slightly uphill, although the actuals of mass and energy requirements are not final yet.

The engine's job will be to maintain the battery at a SOC of 30%, and will do so by continuously matching the average power requirement of the car once it is turned on.  Those energy requirements will roughly be about 8 kWh in the city, and 25 kWh on the highway.

Another interesting note is about the time course of recharging the battery on the road. If one tried to recharge it by maxing engine output, the cells' temperature would get too high, so the idea of rapidly "refilling" it on the fly and then cutting off the generator wont apply. Rather, it seems, the engine will continue to run, constantly matching the needs of the car to keep the battery at 30% until you stop driving.

Interestingly, the motor will likely be programmed to increase rpm when you step on the gas and quiet down when you stop to "simulate" the driving effect people are already used to. This will avoid the sudden unexpected ons and offs.

The graph above illustrated how the pack shall operate.