Funny, I just wrote up the explanation below in response to a similar question on ChevroletVoltage.com ... (dont' bother going there, there really is FAR more activity here, and anything new posted there first usually shows up here within hours anyway)
Some of this comes from a WopOnTour post here a while back.
First, some background. Hybrid vehicles like the Volt and Prius have two separate battery systems: the big, high voltage "traction" battery and the small, low voltage "accessory" battery. The traction battery obviously provides the power to move the car, but it also provides the power to start the gas engine.
During operation, the 12 Volt battery's voltage is maintained by the "accessory power module" (APM) whenever the Volt is "ON", and maintained by the main battery charger assembly (On-Board Charging Module) when the Volt is plugged in and charging is ACTIVE (steady green LED). The APM is a DC to DC converter that takes high voltage (380V?) from the Volt's traction battery and converts it to ~13.0-15.5 Volts in order to maintain the low voltage accessory loads (including the Volt's computers and modules). It also charges the 12V system's battery, also know as an "absorbant glass mat" (AGM) battery. The 12V battery's voltage is maintained when the car is running or charging (by the APM or charger, respectively), but not when parked and unplugged.
Both the APM and the 12V auxiliary battery are located in the rear of the vehicle under the removable rear hatch compartment floor.
Someone had asked a question about jumping the Volt. The scenario that you'd need a jump for is when you've somehow managed to drain the 12V battery (e.g. leaving lights on). In that case, you just need to connect to the 12V battery and give it a little juice, to literally boot up the brains so that the car will start up.
Even if it wasnt (plugged in), you wouldn't be able to drain the 12V battery by leaving a door open, map/dome light on, or even headlights on as after approximately 10 minutes the Body Control Module (BCM) will shut down the various fused feeds to these devices via the inadvertent power relay (aka battery run-down protection).
The Volt is also equipped with a "Battery Saver" mode used for periods of storage. The battery saver mode reduces the parasitic load of some of the electronic modules during long shipment or during vehicle storage conditions. This improves the drain time on the battery (up to 70 days without the battery going dead). (more on this
here)
You can't jump the car if it has a dead traction battery (the HV battery), but it's nigh impossible to run that battery dead anyway. The accessories don't drain the traction battery when the car is parked, and the car won't let you drive it until the traction battery is completely dead -- it stops at 25% SOC (we think) and certainly well before 0% SOC.
The Volt manual has quite a bit of detail on A) jump starting the Volt and B) using the Volt to jump start another car. See pages 10-72 through 10-76. Note that the 12V battery is in the
rear of the car, and you can attach the jumper cables there, but for convenience there are also jump connection points in the
front of the car,
as shown in this post. However you should only use those front points when jumping the Volt; if jumping another car use the rear points. This is probably because of a lighter gauge wire used to feed the front points.