Battery degradation is discussed as a single process. It is several, they operate on different timescales, and user behaviour affects them differently.
Calendar ageing
Capacity loss that occurs with time regardless of use.
Driven by chemical reactions at the electrode surfaces that proceed slowly and continuously.
The rate depends heavily on temperature and on state of charge — a cell stored hot and full degrades far faster than one stored cool and partially charged.
Which is why devices left in a hot car deteriorate, and why long-term storage guidance specifies a partial charge rather than a full one.
Cycle ageing
Capacity loss from charge and discharge cycles.
Lithium ions moving in and out of the electrode structures causes mechanical stress and gradual structural change.
Deeper cycles cause more damage than shallow ones, which is why cycle life figures are quoted for a specified depth of discharge.
Two shallow cycles are generally gentler than one deep cycle covering the same total energy.
The solid electrolyte interphase
A layer that forms on the anode during first use and continues growing slowly.
It is necessary — without it the electrolyte would continuously decompose — and its growth consumes lithium, which reduces capacity.
Which means some degradation is inherent to the chemistry functioning correctly, and cannot be engineered away within the current approach.
Lithium plating
The mechanism behind most acute damage.
When charging too fast, at low temperature, or at high state of charge, lithium can deposit as metal on the anode rather than inserting into it.
That metal is largely unavailable for further cycling, so capacity is lost permanently.
In severe cases the deposits can grow into structures that risk internal short circuits, which is why management systems restrict charging under those conditions.
What this means practically
Avoid heat, which is the largest single factor and is largely within user control.
Avoid sustained storage at full charge, particularly warm.
Avoid running to zero regularly, since very low states of charge also stress the cell.
The middle range is where cells are happiest, which is why several devices now offer charge limiting features.
The measured capacity figure
Devices reporting battery health estimate it rather than measuring it directly.
Estimates use voltage response, impedance and cycle history, and they are approximations that can jump or drift.
Which means a sudden change in reported health frequently reflects recalibration rather than an event.
Replacement
Batteries are consumable and the design decision about whether they are replaceable has become a policy question.
Regulation requiring user-replaceable batteries in portable devices has been introduced in some jurisdictions, with implementation periods running.
Which would change device design substantially and has been resisted on grounds of water resistance and structural integrity.
Second life and recycling
Cells retaining substantial capacity after automotive use can be repurposed for stationary storage, where energy density matters less.
Recycling recovers lithium, cobalt, nickel and other materials, with recovery rates and economics varying by chemistry and by process.
Newer chemistries using cheaper materials are commercially attractive to build and less attractive to recycle, since the recovered materials are worth less, which is a genuine tension in the direction of development.
Chemistry differences
Not all lithium batteries behave the same way, and the differences matter practically.
Nickel-rich chemistries offer high energy density and degrade faster, particularly at high states of charge.
Iron phosphate chemistries offer lower energy density, considerably longer cycle life and greater tolerance of full charging.
Which is why guidance differs by vehicle — some manufacturers recommend regularly charging to full, and those generally use iron phosphate.
Checking which chemistry a device or vehicle uses determines which advice applies.
Fast charging trade-offs
Higher charging rates generate heat and increase plating risk, both of which accelerate degradation.
Active thermal management mitigates this substantially, which is why vehicles with liquid cooling tolerate rapid charging better than those with passive cooling.
For phones, charging overnight at low rates is gentler than repeated rapid charging, and several devices now delay completion until shortly before the usual wake time for exactly this reason.
Measuring degradation
Capacity tests require a full controlled discharge and are rarely performed outside service settings.
Third-party diagnostic tools exist for vehicles and read data the manufacturer's own systems use, which is more reliable than the dashboard estimate.
Safety
Damaged cells present a genuine fire risk, and the failure mode is difficult to extinguish once started.
Physical damage, water ingress and poor-quality chargers are the common causes in consumer devices.
Swelling is the visible warning sign and means the device should be removed from charge and taken for disposal rather than used.
Disposal in household waste has caused substantial numbers of fires in refuse vehicles and processing facilities, which is why dedicated collection points exist.
Warranty terms
Battery warranties typically guarantee capacity retention above a threshold for a defined period or mileage.
Which means gradual decline within the threshold is not a defect, and reaching the threshold is what triggers a remedy.
Remedies vary between repair, module replacement and full replacement, and the terms are worth reading before assuming what is covered.