On Fairphone OS 1.8 this is not required anymore because the bug was fixed.
@Lovisan Which issues exactly are you experiencing with GPS? You didn’t specify in your original post. 
About the battery: At FP they told me that the battery has its own software inside, which sometimes needs to be reset. (This would explain why your bf had problems with your battery and you didn’t with his. Apparently it’s the battery itself which is causing problems.) Since you are in an experimental mood, I might as well share what they told me to solve the issue (I did not have the time to try it yet, I hope it solves my issues too):
- Use the battery up until it is below 0% (until the phone dies).
- Take the battery out of the phone.
- Plug your charger into the phone (still without battery).
- Insert your battery. (This will give the battery a kick)
- Charge your phone in one go until 100%. Do not boot.
If you try it, please share your experience! 
PS.: Thanks you @Sietse for the troubleshooting tips!