I have an old Arduino Fio board that I want to mod to accept a USB-C plug from the existing mini-USB (I don't have many mini-usb cables around). I'm also hoping to make it possible to charge the battery while powering the board, which today fries the board for reasons I don't know.
This is the board: https://docs.arduino.cc/retired/boards/arduino-fio (came as part of the kit for the Safecast bGeige-Nano Geiger Counter).
For the charging+power I'm hitting my own complete incompetence. The manual simply says "The Nano Power switch MUST be turned OFF before charging to avoid permanent damage to the charge circuit." (bg-wiki/NANO-MANUAL.md at master · taroooyan/bg-wiki · GitHub) It really does fry, because I've seen plenty of forum posts various places from people who have tried 
I can't tell, though, whether it's because of the Fio board or because of the circuit of the rest of the geiger counter.
Can anyone here provide me with some insight on why the charging circuit should fry in this case, and whether there is anything I can modify to make that not happen?
One of my plans is to create some form of circuit that would kill the power when the usb is plugged in, but I don't really know how to do that either. Any tips are appreciated. Priority one is to not wreck the board (charge OR power), priority two is continuous operation (charge AND power).
For the plug itself, it seems it should be a relatively simple job to create a converter based on two plugs and extract the correct voltage from usb-c, potentially linked with some protection like MAX4987AE).
Thanks a lot in advance for any help!
Håvard