My understanding is that my Gen 1 Volt mostly uses the engine to drive the generator and only direct couples the engine to the drivetrain above 50mph, presumably for efficiency. If you need to pass it decouples and uses pure electric.
My original point was that the engine/generator is...
I hope it’s like my Chevy Volt. At any point you can switch to the generator to save battery. Plus you can just run it on the gas generator indefinitely by filling the gas tank. Non need to charge the battery on long trips.