Every rechargeable device holds less charge after a year than it did when new. The loss is chemical, predictable in pattern and not reversible by any charging habit.
What a charge cycle actually does
Charging moves lithium ions from one electrode to the other, and discharging moves them back. The materials expand and contract slightly each time this happens.
That repeated movement gradually damages the electrode structure and consumes some of the lithium in side reactions that form a layer on the electrode surface.
Every cycle removes a small quantity of available capacity permanently, which is why capacity is rated in cycles rather than in years of use.
Why heat matters more than anything else
Chemical reaction rates rise sharply with temperature, and the unwanted reactions that degrade a cell rise along with the useful ones.
A battery held hot while charging ages considerably faster than the same battery kept cool, even if the number of cycles is identical.
This is why fast charging, gaming, direct sunlight and charging inside an insulating case all shorten working life, since each raises cell temperature.
Time passes even when nothing happens
Calendar ageing occurs regardless of use, because the same degrading reactions continue slowly whenever the cell holds charge.
The rate depends on the state of charge during storage, and a cell held near full ages faster than one kept part-charged in a cool place.
An unused spare battery therefore loses capacity on the shelf, which is why stockpiling replacements in advance rarely works as intended.
What the device is doing about it
Charging systems limit voltage, taper current near full and pause when cells are hot, all of which slow degradation at the cost of charging speed.
Some devices deliberately hold at a lower state of charge overnight, completing the last portion shortly before the user typically wakes.
Reported capacity comes from software estimating cell health from voltage and current behaviour, so the figure is a calculated estimate rather than a direct measurement.
Why performance changes as well as runtime
An aged cell has higher internal resistance, so it delivers less current under load and its voltage sags further during demanding tasks.
Devices respond by limiting processor speed to avoid an unexpected shutdown, which makes an old device feel slower as well as shorter-lived.
This is why a battery replacement can restore responsiveness in a device whose performance seemed to be a software problem rather than a hardware one.