To each their own, but what I did was to install the backup battery on the Dynon, I boot the Dynon on the backup battery immediately upon entering the cockpit. By the time I'm done with seatbelts etc and ready to crank, it's up and showing all the engine parameters for start - WITHOUT the avionics bus online. Crank/start as normal, then engage avionics and everything else comes up. This gives you a good pre-start check of the backup battery on every flight and avoids having all your electronics online as the alternator comes up.Connect it to the main bus. That way when you turn on the master, you can read the battery voltage. You also want the screen connected to the main bus, not the avionics bus. Do not connect it to the avionics bus.
As soon as you turn on the master switch, the screen will boot up, and you will have everything you need to start the engine, voltage for cranking, fuel pressure for starting, and oil pressure after it starts.
Then turn on the avionics bus for the rest of your avionics.
Good luck.