Just like any other car, if the 12 volt battery is dead, the Volt will NOT start.
However, you could just leave your Volt turned on. Then the 12 volts is being supplied by the traction battery. If the traction battery runs out, the ICE will start.

 

Now, I assume that the 12v in the Volt is there to provide a 12v supply to everything. Has anyone any idea what would happen if this gets almost depleted? Would the Volt start? Should I keep hauling a separate battery?

The 12V AGM RUN/CRANK battery voltage powers some but not all the components in the Volt.

In the Volt, the primary source of @12VDC power is supplied from the (K1) 14V Power Module aka APM DC-DC inverter!

 

Seems like a way to reduce your baggage, so to speak. If you are worried about running the 12 volt down, you could leave the car on. That would allow you to use the high voltage battery and ice if necessary.

 

Start the car after each recharge of the RC batts for five to ten minutes. Should be fine. I haven't used the Volt yet for charging but if I do go racing this summer at our local track, I will charge off the car. Some guys bring generators. No need for that. I have charged off my older car before getting the Volt but my chargers are only 50W. I would be using a 120V inverter off the Volt to power a power strip and multiple chargers. Using a wiring harness off the battery from www.evextend.com

You also could jump start the car using a 4S1P RC battery.

 

Yes, there are others on this forum as well that use the evextend.com inverter kit to give them direct (and easy) 12V access for RC battery charging, turning the car on occasionally as you noted so that it maintains a charge of the 12V battery from the Volt's DC-DC converter.

In short, don't haul a separate battery. Use something like the EV Extend kit and have your car on during these events at least some of the time. The fact that the Volt doesn't need the engine to run when on is a great benefit to take advantage of.

Using an inverter with the kit as it is intended to be used works too, but using the 12V directly if the chargers allow should result in less conversion losses.

 

One way to use the evextend kit for RC is the setup the RC charger to use the 12V output harness rather than a 120V inverter. You can use a high wattage charger, possibly with multiple outputs, and not need to convert to AC and back again.

I like this one as a cheap multi-battery charger. http://www.hobbyking.com/hobbyking/...hobbyking/store/__11170__Turnigy_4x6S_Lithium_Polymer_Battery_Pack_Charger.html
More power: http://www.hobbyking.com/hobbyking/...hobbyking/store/__18812__Turnigy_4x6S_400W_Lithium_Polymer_Battery_Charger.html

Always charge batteries outside the car and in a LiPo sack. You know. The normal safeguards.

 

Easy enough to jump the car should you deplete the 12V. It only needs enough amperage to wake up the main pack.

 

Leave the car running always,
A replacement battery from GM costs about $200
And running it dead or even low lowers it's capacity permanently the more you do it.

 

Got pics of your RC planes...?