How EV batteries age

An EV battery loses capacity gradually through two overlapping processes: calendar ageing, which happens with time, and cycle ageing, which happens with use. How fast each one progresses depends on temperature, state of charge, charging current and cell chemistry.

Two clocks running at once

Battery ageing is usually split into two parts. Calendar ageing continues whether the car is driven or parked, and is driven mainly by time, temperature and the state of charge (SOC) the battery sits at. Cycle ageing comes from actually moving energy in and out of the cells, and depends on how much energy is cycled, at what current and at what temperature.

In a real car both clocks run at the same time, and they interact. Models that combine calendar and cycle components, validated against realistic usage profiles, tend to describe real-world ageing better than either component on its own.

What actually happens inside the cell

At the microscopic level, several mechanisms slowly consume capacity. The most important is growth of the solid electrolyte interphase (SEI), a thin layer on the graphite anode that binds lithium which can then no longer carry charge. Other mechanisms include lithium plating on the anode surface, cracking of electrode particles and structural changes in the cathode.

Researchers often group these into degradation modes: loss of lithium inventory (LLI), where usable lithium is trapped in side reactions, and loss of active material (LAM), where parts of an electrode stop taking part. Different conditions and charging habits push these modes in different proportions.

Gradual, then sometimes faster

Early in life, capacity often drops a little faster and then settles into a slower, steadier decline. Much later, some cells show a 'knee', after which degradation accelerates again. Whether and when that happens varies between cells, even within the same production batch.

Fleet telematics data suggests that many EVs lose on average roughly 1.5 to 2.5 % of capacity per year, with heavier DC fast-charging use and hot climates associated with faster decline. These are fleet averages from a source that is not peer reviewed; individual cars can sit well above or below them.

What this means for you

No single habit decides your battery's fate. Temperature, time spent at high SOC and charging current all contribute, and their relative weight differs by chemistry. Lab results are a useful guide, but vehicle packs have a battery management system (BMS), thermal management and buffers that change how the cells experience your habits.

Voltgevity combines these factors in a model that estimates calendar and cycle ageing from your driving, charging and climate. The result is an estimate with an uncertainty range, not a guarantee.

Key takeaways

Batteries age through time (calendar ageing) and through use (cycle ageing), and both run in parallel.
Temperature, time at high state of charge and charging current are the main levers you can influence.
Fleet averages of roughly 1.5 to 2.5 % capacity loss per year hide large differences between individual cars.
Results from single lab cells do not translate one-to-one to vehicle packs with a BMS and thermal management.