A phone battery holds noticeably less charge after a couple of years of ordinary use. The decline is chemical, gradual and largely unavoidable.
Capacity is lost inside the cell
A lithium cell works by moving lithium ions between two electrodes as it charges and discharges.
Each cycle traps a small amount of lithium in a layer that grows on the negative electrode, and that lithium no longer takes part in storing charge. The electrode materials themselves also change structure slightly with every cycle.
The loss is cumulative and irreversible, which is why capacity falls steadily rather than dropping when something goes wrong.
Heat is the strongest accelerator
Chemical side reactions speed up with temperature, so a warm battery ages faster whether or not it is being used.
Charging generates heat, and so does the processor sitting next to the cell, which is why gaming while plugged in is harder on a battery than either activity alone.
Leaving a phone in a hot car or in direct sun for an afternoon costs more capacity than weeks of ordinary charging.
Time at a high state of charge matters
A cell held near full sits under higher internal voltage, and that stress drives the same side reactions that heat accelerates.
A device that spends every night at full charge therefore ages faster than one that spends those hours part-charged.
This is why recent phones learn a charging pattern and hold at a lower level overnight, completing the charge shortly before the alarm.
Fast charging trades speed for life
High-power charging pushes current in quickly, and both the current and the heat it produces add wear.
Chargers manage this by slowing sharply as the cell fills, which is why the first portion of a charge is much faster than the last.
Occasional fast charging has a modest effect; making it the only method a device ever sees has a larger one over the life of the phone.
The reported percentage is an estimate
A phone cannot measure stored energy directly. It infers charge from voltage, current and temperature using a model of how the cell behaves.
As the cell ages the model drifts, which produces sudden drops, a reading that sticks, or a shutdown with charge apparently remaining.
Recalibration routines correct the estimate rather than the battery, so the reading becomes more honest while the underlying capacity stays exactly where the chemistry left it. A phone reporting a lower maximum capacity after an update has usually just measured itself more accurately.