I don't own a car charging adapter, but on most trips where I'm using my phone for navigation I'll also have a laptop and charger cable in the car.

For Apple laptops, if you start charging while the computer is awake it will keep charging when the laptop is sleeping. For PCs it varies by manufacturer, but IIRC some of them have a BIOS/UEFI option for powering USB ports while they're asleep.