Twice my Fairphone 3+ has turned off despite having battery. It is unresponsive until I have charged it for some minutes. The charging light comes on instantly, but it will not turn on until I have charged it for some time. What can be the cause of this? The phone is about 28 months old.
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If cleaning the battery and phone contacts does not help I assume your battery is bad.
You can take it out and check if it’s bloated, place it flat on a table and try if it spins freely once you give it a slight turn. If so it should be replaced.
Anyway it could be damaged internally. Maybe keep an eye on at which power level your phone switches off or if the level decreases quicker when it’s going to reach a low value.
I once had a FP2 battery that would only last until ~30% before the level decreased per second until the phone shortly switched off. So the max. battery capacity was only about 70% remaining.
At an age of about 28 months noticeable degradation could have happened depending on your individual use.