After many years, I'm getting back into some simple wiring. I'm trying to make a self-sustained wifi tracker. This tracker will have a 3.3v Button cell lipo battery as a battery backup and I want to be able to charge this battery backup any time it's plugged into the main battery. The main battery can be up to 42v, and needs to be stepped down to 3.3v to charge the button cell without damage and also to run the ESP8266 wifi module.
In the diagram below, I'd like to be able to shutdown the pololu step-down converter when the button cell lipo is charged, but I don't know what this is supposed to look like. Possibly an N-channel mosfet?
I'm hoping this circuit isn't completely wrong in the first place, but if it's not - how do I use the SHDN (shutdown) pin on the Pololu to not allow any more voltage to come through? I feel like I need a switch behind the button cell to turn off when the button cell is 3.3v or more?
Edit: The reason I want the button cell in there is incase the main battery falls off or is disconnected in any way. Also, this will be "always on", so when it IS hooked to a battery, it will charge the 3.3v button and "turn back on" if it has previously died.