Concur with the above statements. GRT EFIS devices are very simple - they are ON when power is ON. (One minor exception is the Mini series with internal battery but that's a totally different kettle of fish.)
If you are powering via both main bus and backup battery then you'll need to rethink how you have made those connections.
I have three GRT EFIS devices in the airplane. Two of them are wired to a backup battery. This backup battery is charged from the main bus.
Schematically power runs from main bus through a breaker and diode to the backup battery. Power then runs from the backup battery through a fusible link to a SPDT switch, then to the Essential Bus. From the Essential bus power flows through individual circuit breakers to the EFIS devices.
Primary power for the EFIS devices comes from individual circuit breakers installed on the main bus.
In day-to-day operation this setup is very simple to operate. Get in the airplane, flip on the ESS BUS switch, the EFIS devices (and EIS) come alive. Go through all the pre-start checklist, flip on the Master switch to energize the Main Bus. The EFIS devices are now powered by the main bus. Crank the engine - if cranking current draws down the main bus the EFIS devices don't even blink because they have access to the ESS Bus power. In the event the main bus goes dead, the EFIS and EIS keep running until the ESS battery depletes.
Hope this helps.