The 12V converter charges the 12V battery from the 245V battery pack. I have no idea if the TCH 12V battery has a longer life than a traditional 12V starting battery. Like any 12V lead acid battery, it will fail early if run flat (map/trunk lights left on.)

If the 12V battery dies, only a few amps of power is required to "jump" the TCH. The battery only needs to start up the TCH computer. Once the computer is running, the HV battery powers the TCH and starts the ICE.

If the there is not enough power to "boot up" the TCH computer, I imagine you would know it right away. Only a few amps are needed, and if not present, I doubt the power door locks, radio or dash lights would work either.

Edit: relays disconnect the HV traction battery when the TCH is off, for safety and to prevent parasitic loads from draining the HV battery. I just came back from a 23 day vacation, and the TCH started right up. No problem with the 12V or traction battery.