I meant in terms of form factor - make sure it's a pouch cell because 18650s will be way too big to fit in the Gameboy shell. Which is kind of the same as 'find one that's small enough', but it lets you filter out a whole category of power banks.
Most charger/batteries are 3.7/4.2 V (that's nominal voltage/max voltage) and you can charge a 3.8v/4.35v cell with a 3.7/4.2 charger but not the other way round.
Otherwise the battery will burn/explode/vanish?
I am new to this, I think I'll have a look at electronic courses, it's been 20 years since I last saw a diode and whatnot...
Hello,
I have made 4 pizero boys and all 4 have used a battery bank much like the one you pictured. I wired directly to the pins of the usb port and connected those to my pi for power. I disassembled the housing for the battery itself and it fit nicely in between a gameboy game shell if you remove some of the inside of the shell. I then just had to find a nice spot to line up the micro-usb port to be able to charge and chose the link cable port. Not sure if this will help, but I have had success with a portable power bank.