Originally Posted by BCP Boys
I have a similar setup. I think I understand your last post but a drawing would be better to understand what you are trying to do. On my system, I have a traditional dual Cessna switch that control 2 separate batteries, a separate switch that turns on and off my back up battery for the G3X system and an alternator switch. The backup battery switch is the first thing I turn on and the last thing that I turn off. It needs to be On to re-charge the backup battery (at least in my set up it does). The only thing I will emphasize is this, if I'm understanding you correctly you want the switch to be OFF and for it to still work as outlined above... It needs to be On for it work this way and also to charge the backup battery... otherwise, why would you even have a switch to the backup battery?
I stink at drawings. Maybe I stink at descriptions, too. The switch would have three positions. From bottom to top, they would be: OFF, which disconnects both the master and the backup battery. BACKUP, which connects only the backup battery. And MAIN, which connects both the backup battery and the main battery. It would not
be possible to turn the backup battery off while the main battery is on. It would be possible to turn the main battery off while the backup battery is on.
But the point immediately below yours is quite valid: In turbulence, it would be quite easy to accidentally push the switch full down and shut off the entire airplane while trying to switch to backup power in the event of a main power bus failure. Separate switches are probably safer for that reason. In my plane, when all else is equal, safety makes the decision.