Just curious how these Volts are powered.
Just as background, because it will probably matter sometime...
12v powers the car when it's "off". The high voltage (HV) battery output is completely disconnected. You need that 12v power to handle starting the car, or to start charging the HV pack from the wall. In the 2013 (I'm pretty sure), the wall charging does not charge the 12v -- it only charges when the car is on.
Starting the car requires several seconds of very even power from the 12v, to start up many computers in the proper sequence. The startup sequence is quite sensitive to power fluctuations, so a weak battery will cause mysterious gremlins that look like things broken but are just "didn't get enough 12v power at the right time". Replacing the 12v fixes a lot of weird things, especially ones that don't repeat from day to day. Once all the computers are started up and working properly, then the contactors for the HV pack close and power feeds to the Aux Power Module (APM -- the Volt's "alternator"). The APM will take over powering the low-voltage parts (fans, lights, computers, pumps, etc) and first check, then start charging the 12v battery. That battery check before charging is because reviving actually dead AGM batteries is very tricky, and prone to going wrong in Very Bad ways. If the 12v is too dead, the APM will not charge it.
The 12v does not start the engine. The engine is started by the secondary motor/generator (MGA) turning the engine to get fluids flowing, then spark is applied. You have a 55kw starter motor. The engine WILL START if it's able to run at all.
Since the 12v doesn't start the engine, and the car-starting sequence is so sensitive to power problems, it's often difficult to tell when the battery needs replacing. Unlike regular cars, the Volt's manner of using the 12v means hot climates (like San Diego, Arizona, New Mexico, Texas) tend to be much harder on batteries than cool ones. It's not unusual for a Volt AGM to need replacement in four years in a hot place, and other Volts tend to need replacement 12V batteries in 6-7 years. (Exceptions either way happen, but a LOT of people with early Volts have had to replace theirs.)