This #wiki will explain what to do if your battery is completely empty - how to get the battery to charge again if there is no hardware fault and how to test what hardware might be at fault.
For different battery issues see the general #batteryguide.
Please note this wiki was written before FP3(+) and FP4, so not everything might apply to FP3(+)/FP4. E.g. when the FP3 is plugged in without battery nothing happens. Also Quick charge protocols work different and I would not take the time noted below for granted, e.g. it might take longer than 15 mins to turn the LED yellow etc.
What should happen if I try to charge a deep-discharged battery?
If your battery is deep-discharged (completely empty) you should see a red-blinking LED if you plug in your phone.
Keep it plugged in and don’t attempt to turn on the phone.
After a maximum of 5 minutes the LED should turn solid red, then the phone should vibrate and the screen turn on. It will then display the charging screen for a while until the screen turns dark again. Then the screen will remain black and the LED solid red.
Keep waiting until the LED turns yellow (or even better green). It shouldn’t take longer than 15 minutes to turn yellow and the phone shouldn’t get very hot.
If that’s what happens everything is fine. 
What should I do if it doesn’t work like that?
Take the battery out and let it cool down for 30 minutes (or some minutes in a fridge - not freezer).
Meanwhile plug in the phone without the battery for a while. You should see the following behavior:
- the phone vibrates
- the display shows the Fairphone logo for a second or so
- the LED lights up for another second or so
- this happens over and over again
→ If this doesn’t happen, then some part of your hardware is not working (most likely the bottom module or the charger or USB cable).
Try the same with different chargers, cables, plugs & wall sockets. If no combinations bring different results #disassemble the phone and see if cleaning the contacts between the bottom module and the main module changes something. If it doesn’t than you’ll most likely need a new bottom module.
→ If this does happen then it means the phone’s hardware should work charging-wise. There are cases however where the charging is slower than the drain e.g. during the boot-process. (digression: With a turned on phone you could see this in the settings. If the LED lights and the status bar symbol shows the battery is charging, but in Settings > Battery it says “not charging”, then charging is insufficient).
If you now put your cooled down battery back into the phone and try charging again as explained in the first chapter and it still doesn’t work, then there is probably still something wrong with the hardware. In this case it’s most likely the battery.
How do I know which part really needs replacement.
You can only be absolutely certain by meeting another Fairphone owner and swapping battery and bottom module one by one to see which part is at fault.
See if there are #fairphoneangels near you - they’ll gladly help test your hardware.
If you don’t have access to someone else with an Fairphone, but still have warranty (1 year on the battery, 2 years on everything else) contact support and describe everything you’ve tested and tried so far. They’ll send you either a free battery or bottom module - depending on what they deem most likely to be faulty.