A two-year-old phone runs down by mid-afternoon on the same routine that once lasted all day. The battery has physically changed, and the rate of that change depends mostly on heat and cycle count.
Capacity loss is a chemical process
A lithium-ion cell stores energy by moving lithium ions between two electrodes. Each transfer is slightly imperfect.
Over time a layer builds on the anode surface and some lithium becomes permanently locked in place, unavailable for future cycles.
The cell is not being throttled or artificially aged. There is measurably less material available to carry charge than there was when it was new.
The unit of wear is the full cycle
Degradation tracks total energy moved, not the number of times a cable is plugged in.
Two charges from half to full count as one cycle. This is why a person who tops up constantly is not necessarily wearing the battery faster than someone who charges once a day.
Manufacturers rate cells to retain a defined share of original capacity after a stated number of cycles, which is why capacity health is reported as a percentage of the original figure.
Heat accelerates everything
Chemical reactions speed up as temperature rises, and the reactions that degrade a cell are no exception.
A phone left on a dashboard, used heavily while fast charging, or kept in a case during a long gaming session is ageing faster than the cycle count alone suggests.
Sustained high temperature is the single most damaging condition a battery encounters in normal use.
Sitting at full charge is its own stress
A cell held at maximum voltage is under strain even when idle. Overnight charging keeps it there for hours every night.
Optimised charging features exist for this reason. The phone learns the alarm time, charges to around eighty percent, and completes the last portion shortly before waking.
Storing a device long term is best done near a middle state of charge rather than full or empty.
Why the reported percentage drifts
A phone estimates remaining charge from voltage and usage history rather than measuring it directly. As the cell ages, that estimate loses accuracy.
Sudden drops from a comfortable reading to shutdown usually mean the estimate has drifted rather than that the battery failed instantly, and cold weather makes the same effect appear temporarily.