Electric car batteries lose capacity over time, but not in a straight line. The curve has a steep early section and a long shallow one, and understanding why explains most of what owners observe.
What capacity loss means
A cell stores energy by moving lithium ions between two electrodes. Capacity is the quantity of ions available to make that journey, and degradation is the gradual removal of some of them from circulation.
Ions are not consumed by being used. They are lost when they become chemically bound in places from which they cannot return.
Separately, the cell's internal resistance rises, which reduces the power available even when stored energy remains.
Why the first drop happens quickly
During the earliest cycles a protective layer forms on the negative electrode, built from electrolyte reacting with the electrode surface.
That layer is necessary because it prevents further uncontrolled reaction, but forming it consumes lithium, which is taken out of circulation permanently.
Most of this happens early, which is why an owner may see a few percentage points of capacity disappear in the first year and then very little in the next several.
What drives the slow phase
After the initial layer stabilises it continues to thicken slowly, consuming a little more lithium as it does. This proceeds with time rather than with use.
Temperature governs the rate. Chemical reactions accelerate with heat, so a battery kept hot ages faster than an identical one kept cool, regardless of mileage.
Electrode materials also degrade mechanically, as repeated expansion and contraction during charging causes gradual cracking of the particles.
Why charging habits matter
Holding a cell at a very high state of charge keeps its voltage high, and high voltage accelerates the reactions that consume lithium.
Very low states of charge stress the cell in a different way, which is why manufacturers advise keeping a battery within a middle band for long-term parking.
Rapid charging generates heat and forces ions to move quickly, which is manageable occasionally and more significant as a daily routine, particularly in warm weather.
How manufacturers slow it down
The usable capacity presented to a driver is deliberately smaller than the pack's true capacity, so the cells never reach their genuine limits.
Liquid cooling keeps cells within a narrow temperature range during driving and rapid charging, which addresses the largest single accelerant.
Battery management software also balances cells against each other, since a pack ages at the rate of its weakest cell rather than its average one.