Several people have been working of similar stuff:
Here was a discussion on how to connect n extra battery to the USB debug connector
Doubling the battery capacity by soldering two stock batteries together has been tempted here. BUT this has one huge safety drawback as neither the battery Protective Circuit Board (PCB) nor the FP charging circuit will be designed to handle higher load. And, most importantly, if you connect two battery cells together without charge balancing, you may end up with thermal runaway (if one battery charges the other one because of slightly different voltage, you will burn both of them), which might lead to serious injury (see here).
@jayy has been working on a circuit for using standard 18650 cells over here, but I don’t know whether that project has been continuedhttps://forum.fairphone.com/t/2-batteries-in-parallel-in-a-new-case/49374/2
@ElKrasso sure I didn’t optimize visual design, as I first and foremost needed a working prototype in order to get more juice into my FP2. If you’re in for remixing my design ( all files are posted on thingiverse), go ahead, I’d be pleased printing your version!
Btw I’ve been gathering charge stats for several months now. I’ll try to compile and analyze them within the next few week to see how much this actually gained, but roughly I got 2,5 original charges with the battery extension.
@urs_lesse : no wiring needed (if you manage to not cut the wires betwen both boards when unmounting them from the original case). The electric circuitry is kept from the original.