I see. Does the generator drive the vehicle, or just recharge the batteries?
That I do not know.
My guess would be there’s some sort of electronic controller which directs power to the drive-motor to the extent it’s needed to respond to the “gas pedal”, and whenever excess power is available charges the batteries.
Or, maybe the batteries are disconnected entirely when the aux engine is running.
The latter would be far simpler/cheaper.
Well... it works like a diesel electric locomotive.
The batteries are charged while in the garage to about 90% or so capacity and then you drive till they hit 30% capacity and then the gas motor kicks in to give power to the electric motor while the battery acts as a surplus charge for extreme driving conditions.
It’s ultimately the best of all worlds.
The generator operates at 99% efficiency (most regular autos run theirs at about 20-30%) thus giving it about 50mpg as opposed to 30mpg.
The charge control keeps the battery at optimal cycling(and has a built-in heatsink to regulate battery temp) so it virtually guarantees a 5-10 yr life cycle on the batteries.
And the use of electricity rather than gas makes it more efficient on the whole. That being rather than using x amount of watt-hours to go x miles you use the same amount to go 4-5x.
Really the estimates are that at worst, with a dead Volt battery you’re stuck going 70mph or so and maybe 55mph on a slight incline.