@Koepel
I mean having two voltage regulators, each 5v but only one would ever be in use at a time.
This works on my teensy 4.0, where when its plugged into my laptop I can write to it and it powers my whole pcb. If I connect a battery (With the USB removed) and it runs the code perfectly. You can physically cut a connection on the teensy in order to have a serial output and have it powered by a battery at the same time.
I'm just wondering how I could implement this.
Luckily all I did was copy the pins for the boot loading 
Thank you again - Harvey