Hi and welcome to the community forum.
Assuming that this is not just a discharged battery (I would recommend never letting it discharge below 5% and in normal times try to keep the battery charged between 20% and 80% …) …
In a case like this your first action should be to report the problem to official Support (see: contactsupport ). Also make sure that you register your phone so as to benefit from extended warranty in the future.
As to finding which component is at fault, you may be able to find a Fairphone angel near you who could help with parts from another phone for testing.