My Fairphone 3 (FP3) has also recently, suddenly, and unexpectedly developed a similar-ish problem. It seems to have occurred in five phases:
Phase I: The phone was purchased shortly after FP3 was released, and so is quite old; the battery is the original. It has been “intensively used” (charged daily(-ish)) all these years, so I presume the battery is nearing EoL (End-of-Life); indeed, a fully-charged battery does not seem to last as long now as it did originally, albeit I also use the phone much more nowadays (mostly due to the pandemic).
Phase II: Several weeks ago ago — before the upgrade to Android 11 — the battery began draining fairly rapidly, barely lasting “a day”. In addition, whilst previously the phone would auto-shutdown at c.1% remaining charge, it began shutting down at c.20% remaining charge.
Phase III: Shutdowns now happen at c.80%(!) remaining charge. However, if connected to an external power source (battery pack or A/C mains), worked fine. Obviously not correct, so disassembled the phone, checked for dirt, etc., found nothing, reassembled, and the phone seemed to start working “normally” (see Phase I, above). (This was the first disassembly since installing the FP3+ Camera modules at about the start of the pandemic; i.e., in about three years.)
Phase IV, “Catastrophic failure”: Again, shutdowns at c.80%, but this time an external power source was not a work-around, the phone was oscillating every few seconds between charging and discharging! Disassembled the phone again, and almost found nothing — no dirty contacts, etc. But I did notice the phone came apart much more easily than it should have, suggesting it was not firmly reassembled during Phase IV; this time, pressed everything back together again very tightly.
Phase V (current situation): Seems to be ”Ok” (i.e., back to Phase I), albeit insufficient time, etc., has passed to conclude this definitely.
Conclusion? Puzzled. My current best guess is the battery is ageing (“failing”) and then became “loose” (poor contacts).
Other than obtaining a new (replacement) battery, any suggestions?
Thanks for any constructive advice!