With thanks to the battery guide.
A battery reset is also called a Kickstart.
First method (FP2 and FP3):
- Let the battery run all dry (ignore all battery warnings until your phone switches off by itself).
- To kickstart the battery: Take out the battery of the phone; Wait for half an hour and put the battery back in the phone;
- Connect the Fairphone to a charger; If the charging light is blinking, it means that the battery was totally empty, wait for some minutes until the red light becomes fixed;
- A loading battery icon should appear for a few seconds (a short press on the power button will display it);
- Please keep on charging for more than an hour because the battery was likely totally drained. You might need to do it twice - or at least let it shut down by itself twice - before the report is better.
- If the above doesn’t help, try the second method.
Second method (FP1, FP2 and FP3):
- 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).
- [Wait a few minutes]
- Insert your battery. (This will give the battery a kick)
- Charge your phone in one go until 100%. Do not boot.
More info about the battery, see the #batteryguide.