I got this directly from an engineer at Chevrolet.
How is the 12v battery is charged?
The Volt does have a 12V battery, just like any other vehicle on the road. It is used the same way in the Volt as in other vehicles as well - that is to power electronics, computers, and accessories within the car. For example, things like radios, power windows, etc., are all powered off the 12V battery. It is only the propulsion battery & drive motors that are powered by the high voltage battery. The 12V battery is actually not used to start the engine/generator – the vehicle uses the high voltage battery to do this.
When do you know if the 12V battery is dead?
Just like any vehicle, you would know if the 12V battery is dead because nothing would work. Things like power locks, windows, radios, etc., would not function. Although we do not use the 12V battery to start the gasoline generator, it cannot be started either, because all of the computer & control systems in the car are powered from the 12V battery, so if they cannot be powered, then the engine cannot be started.
Does it charge while the vehicle is moving?
Not in the sense that vehicle motion puts energy into the 12V battery, but yes, the 12V battery is charged whenever the car is "on". Again, it helps to compare to a gasoline vehicle here. In a gasoline vehicle, the battery is charged when the engine is running (by the alternator). In the Volt, whenever the vehicle is "running" (i.e. powered on), the 12V battery is being charged, although not by an alternator, but rather by a converter that takes power from the high voltage battery.
If I leave my volt plugged in and leave my lights on will it drain the 12v ?
This depends on whether the car is "on" or not. Again, think of a gasoline car. If you leave it parked, with the engine off, and the lights on, then your 12V battery will be drained. If you leave the engine running, then you will not drain the 12V battery (as the engine will keep it charged). Same is true in a Volt. If the car is "powered off" and you leave the lights on, this will drain the battery. If the car is "powered on" then the battery will be kept charged.
Does the 12 volt slave off of the home charger if needed?
No, it does not. The charger that charges the 16kWh traction battery converts 120V AC or 240V AC into high-voltage DC to charge the Lithium Ion propulsion battery. There is a separate module in the car that charges the 12V battery. This "Accessory Power Module" converts high-voltage DC (from the 16kWh battery) into 12V DC (for the 12V battery). This module is only "turned on" when the car is "running" or powered on. (again, think, just like an alternator in a gasoline vehicle - it is only "on" and producing power when the car is running).