18650
http://www.ebay.co.uk/itm/18650-Li-ion- ... SwWTRWw57q

Dimensions 18mm x 65mm 3.7v
I think these are the type of battery cells used inside laptop batteries. They're lithium Ion, and just one of them can be 6000mAh, I already have a powerbank using them and they work very well, is there some way to adapt these for the gameboy zero project? I think with a squeeze you might fit a couple in a slightly adapted gameboy case.
But I'm more interested in firstly how to adapt a couple to have a JST-PH connector to work with the rest of the project. Any gurus know how it might be done?