Does anyone know of, or use, a backup power supply (i.e. rechargeable battery) for their avionics. The reason I'm asking may be obvious, but I'll state my rationalle nonetheless: I'm having a GTX-345 transponder installed and plan on using it with Foreflight on an iPad. I could use the Foreflight synthetic vision, Artificial Horizon, and GPS location as backup instruments in case of a major electrical/avionics/instrument failure. However, these features rely on input from the transponder. Therefore, a backup power supply for the GTX-345 could save the day.

For us, our gyro instruments (AI and DG) are both vacuum, so an electrical failure will not affect our ability to keep the plane upright and fly headings. We have a GTX-34, thus we have AHRS to ForeFlight for Synthetic vision as backup for the vacuum system.

We have a GNC-300XL as our primary GPS/com. Garmin did make a backup battery for the GNC-300XL but it was not installed in our A/C when we bought it. Since the GNC-300XL is a first generation IFR GPS, we figured putting adding $$$ into supporting it was probably wasteful, plus we were doubtful whether we would be able to source one. We also didn't to add the weight of the backup battery.

We have ForeFlight and dual VOR/ILS as a backup to loosing the GPS for navigation.

Our solution for backup of the electrical system was a Stratus 2. It will provides GPS to ForeFlight and had AHRS for ForeFlight Synthetic Vision. It also doesn't weight much. We did install USB charger ports so we keep the iPad, Stratus and our cell phones charged enroute. Should we experience a complete electrical failure, our plan is to declare an emergency, use the fully charged Stratus to navigate to the nearest safe airport.

why,don't you upgrade the entire panel and have a great aircraft that will do everything.

I forgot to mention that the Stratus 2 (now 2S I believe) also has ADS-B/TIS-B In.

Though you might not receive as much traffic info during an electrical failure, with the loss of the GTX-345 ADS-B out, ADS-B/TIS-B In is better than nothing.