How to pick a voltage comparator switch for a low power circuit
A comparator is indeed a great solution for this, given that is pretty much a comparator's only job. The transistor solution will be much more difficult to tune and make stable. By the time you've added enough to the transistor circuit to get it to behave well, you'll have effectively designed a comparator.

To keep things really simple, consider a 5-pin, push-pull output, comparator IC like the LMC7211. Your requirements will be met by a huge range of comparators, so I'll just use the LMC7211 to explain.

Simply connect your scaled battery voltage to the inverting - input pin, your threshold voltage to the non-inverting + input pin, and your 3.3V supply to the supply pins. Finally, because the battery voltage changes very slowly and will likely increase at the moment you turn off the car, you'll probably want to add some hysteresis. Hysteresis means the turn on threshold is different from the turn off threshold so the circuit doesn't bounce repeatedly between on and off. To do so, just add a large value resistor from the output pin to the non-inverting input pin. The output pin will then give you the signal you require.

Something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

With the resistor values I've used in the schematic, the output goes to 3.3V when the input falls below 1.76V and the output goes to 0V when the input rises above 1.94V. Adjust to suit your preferences.