Hi
what I have used the 2 year is wery simpel.
First I use a Futaba Fastesst reciver ,Jeti Mezon with bec( dont know if other ESC will work like this).
I use a simple 2S battery(no regulator) ca 200mah as backup Power this is just conneted directly to RX only a simpel swithc between, the idea is that the BEC in the Mezon hold the backup battery at 8V at all time , i know this is a bit lower than fully charge but will give me Power to land if the BEC or main battery fail.
Routine on the field is like this first Power up the TX then the RX from the backup battery then arm the main battery and fly when I land i turn off the main and then the backup.
I have newer charge the backup just Connected it after i bougth it and that is all.
I have newer had to use it so far but this simpel system has worked fine for about 200 flights.
If you Connect this to others ESC with Bec be sure that it will work with power from the backup battery