Those who follow this site closely should have a good understanding of the relationship between the generator and the battery in the Chevy Volt. Unfortunately Edmunds did not, stating that the engine does not recharge the battery, and created a bit of an Internet frenzy over, well, nothing, they were just wrong.
The battery is recharged by the engine, but only as little as possible. The point is to avoid petroleum use.
Here's how it works.
For the first 40 miles, the fully charged battery (80% state of charge) powers the electric motor. Regenerative braking can help recharge the battery to a certain extent.
When the vehicle drives past 40 miles, the battery reaches a 30% or so state of charge. So called "the customer depletion point", preventing the battery from going below that sustains the batteries longevity. Lithium-ion cells don't survive as long when they are deep discharged, and GM has the goal of 10 years/150,000 miles.
At the customer depletion point the combustion engine fires up and operates at one of several optimized fixed RPMs. The engine turns the generator, producing electricity.
The electricity, at around 50 kw, serves primarily to propel the car. As the driver drives, there may be times when more electricity is made than the motor needs. Guess where that goes? The battery of course. GM doesn't want to waste energy or unnecessarily use petroleum. As well the regenerative brakes opportunistically may put charge back into the battery.
The generator doesnot fully recharge the battery. It doesn't make any sense to, for then you would be using petroleum to travel. The goal is to use the electric grid, so the car will carry on at approximately that 30% state of charge until the driver can get to an outlet and then fully recharge. Without recharging at an outlet, the car could drive indefinitely using gas and refills, but doing so would defeat the whole point of petroleum displacement. It may be useful on the occasion when a long continuous drive is necessary though.
There may be times when generated electricity so far surpasses the needs of the car (i.e. a long downhill), that the battery level might get high enough to let the ICE cut off, but this probably wont be often. If it does occur, then as the drive continues and the 30% level is reached again the ICE will turn on again continuing the cycle.