I asked Chevrolet specifically for an answer to these questions and they sent it to Volt Engineering and this is the reply. They knew that it would be posted.
I would agree that the 12 volt battery should be charged with the car but it is not. It is charged while the car is operational. Since the generator starter uses the main battery; I do not think they should even have a 12 volt battery (personal belief).
Regardless; we all need to know how this battery works so that we can manage it. It would be fun boosting an electric car