Rich, the diagram I posted earlier includes a map switch circuit specific to the EDIS-based ignition I've been flying, as well as a cheap voltage monitor sourced overseas. It is unnecessarily complicated for your Surefly application.
Given you have two alternators, a power supply for the second Surefly could be simplified. Mostly you just want to be made aware if the IGN2 battery is disconnected from the alternators and primary battery. B&C sells a nice little voltage monitor...
https://bandc.com/product/over-under-voltage-sensor/
...which will flash a panel LED when the feed to Surefly #2 drops below 12.5V.
Note a few conceptual points. The little backup battery is truly just a storage vessel. During normal operation, all the electrons source from one of your alternators. Take the alternators off line, and both ignitions still access
both batteries. You have the option of shutting down one ignition, doubling your ignition range. Worst case, you could throw both alternators and the main battery over the side, and still fly an hour or two on a 3 AH backup.
Note the fusible links in the ignition feeds. They are real, real reliable, and can be located right at the battery terminal so every inch of hot wire is protected. Less wire length, no breaker or fuse block. Components not installed never fail.
BTW, money, mouth, yada, yada....I have failed an alternator over the Smokies and continued to south Alabama without concern. Main is a PC925 (I need the mass in the back in a 390 powered -8), backup is a little 3.8AH.
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