Pixhawk Backup Power

I am fitting a pixhawk on a solar boat and as a backup to the main battery i was going to connect an auxilary battery up to power the pixhawk. can anyone see any issue with using the usb power supply rail as a backup 5v supply to keep the pixhawk alive and nothing else? my understanding is the pixhawk has 3 independent power rails, one from the servo rail, one from the power module and one from the usb, so if i connected a 5v supply it should keep only the pixhawk and gps alive in theory.

That’s fine from the wiki:
Pixhawk can be triple-redundant on the power supply if three power sources are supplied. The three rails are: Power module input, servo rail input, USB input.

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