Why do lithium batteries lose capacity over time?
Mostly because a passivating layer keeps growing on the negative electrode and permanently consumes lithium — every charge cycle loses a little, heat and high states of charge make it worse, and none of it is reversible.
The SEI layer
On first charge, electrolyte reacts at the graphite surface to form a solid electrolyte interphase. That layer is necessary — without it the electrolyte would keep decomposing — but forming it consumes lithium, and it keeps thickening slowly for the whole life of the cell. Every bit of lithium locked into it is capacity that never comes back.
Why heat is the main accelerant
The reactions that grow the SEI are chemical, so their rate roughly follows Arrhenius behaviour: warmer means faster, sharply. A cell held hot ages measurably faster than an identical one held cool, even doing nothing — calendar ageing is real and it is mostly temperature.
Why a full charge is harder on it
A high state of charge means a high electrode potential, which drives electrolyte oxidation at the positive electrode and lithium plating risk at the negative. That is the reason charge limits exist in phones and cars: storing at 100% is a stressed state, not a neutral one.
The other mechanisms
Mechanical stress from repeated expansion cracks particles and exposes new surface, which grows more SEI. Transition metals dissolve from the positive electrode and poison the negative. Lithium plates as metal if charged fast when cold — which is both a capacity loss and a safety concern.
What actually helps
Keep it cool, avoid sitting at very high or very low charge for long periods, and avoid fast charging a cold cell. Full cycles are less damaging than the folklore suggests; heat and time at 100% are worse than the folklore suggests.
Why the reported percentage is not the whole cell
A battery management system reserves capacity at both ends and reports a percentage against that usable window rather than the full range of the cell. A pack showing 100% is not fully charged in the chemical sense, and one showing 0% is not empty — the reserve exists precisely because the extremes are where the damage happens. It also means widening that window in firmware is trading range now for degradation later.
Sources
Vetter et al., Ageing mechanisms in lithium-ion batteries (J. Power Sources, 2005); NREL — battery lifetime and degradation. Every claim above is drawn from these; where a claim is contested or unsettled the article says so rather than picking a side.
How this article was made
Written by a person. Reviewed by Editorial review, Leumas on 2026-09-02. 379 words.
Questions
Why do lithium batteries lose capacity over time?
Mostly because a passivating layer keeps growing on the negative electrode and permanently consumes lithium — every charge cycle loses a little, heat and high states of charge make it worse, and none of it is reversible.
How do I know this is right?
The sources are listed on the page and a person reviewed it on 2026-09-02. Where a question is genuinely open, the article says so instead of resolving it.