Why batteries age

Time, cycles, temperature and charging explain lifespan better than a magic expiry date.

A battery ages even standing still

What happens inside the cell with every charge

Each charge-discharge cycle moves lithium ions between cathode and anode through the electrolyte, and that repeated movement physically alters the cell's internal interfaces. Some of these side reactions are harmless or even beneficial — they form, for example, a thin stable protective layer on the anode during the first cycles —, but others slowly consume active lithium that can no longer take part in useful reactions, thicken internal layers until they hinder ion flow, chemically degrade the electrolyte, or mechanically damage the crystal structure of the electrode materials. The sum of all these effects, accumulated over months or years, is what shows up externally as loss of capacity and power.

Two clocks running at once, not just one

Battery degradation proceeds through two independent mechanisms that add together. Calendar aging happens simply with the passage of time, whether or not the battery is actively used, and is markedly accelerated by high temperature and by keeping the cell at extreme states of charge — very close to 100% or very close to 0% — for long periods. Cycle aging, on the other hand, depends directly on how much energy flows in and out of the cell: the number of full cycles, the depth of each discharge, the power demanded during charge or discharge, and again temperature during those processes. A battery that is barely used but stored at 100% charge in a hot environment can degrade almost as much as one used intensively under moderate conditions — the two clocks run in parallel, they do not substitute for each other.

"Useful life" is a threshold, not an expiry date

It is easy to imagine a battery "dying" at a specific moment, as if it had a factory-stamped expiry date. In reality, useful life is a threshold defined for a specific application, not a fixed physical property of the cell. An electric vehicle may start showing a noticeable range loss when the battery retains, say, 80% of its original capacity — a threshold the driver experiences as a real limitation for long trips. That same battery, however, can still have plenty of capacity left for a less demanding stationary use, such as storing home solar electricity during the day to consume it at night, provided a proper diagnosis confirms it is still safe and the economics of that second application make sense. This is exactly the principle behind battery "second life," developed further in this section's recycling leaf.

Why there is no universal number of years or cycles

Any generic figure claiming "this battery lasts X years" or "Y cycles" should be read with scepticism if it does not specify the exact conditions. The specific cell chemistry — LFP versus NMC, for example, with very different degradation profiles, developed in this section's battery-types leaf —, the whole pack's thermal design, the quality of the battery-management software, the climate where the vehicle or equipment operates, and each person's real usage pattern substantially change the outcome. Two factory-identical batteries, subjected to different climates or charging habits, can end up with notably different degradation results after the same number of calendar years.

Making it last without turning it into an obsession

  • Avoid prolonged heat and follow the manufacturer's thermal recommendations.
  • Do not keep it at maximum charge for long periods if the equipment allows a limit and you do not need the full range.
  • Avoid habitually discharging to the extreme when possible.
  • Prefer moderate charging for everyday use; reserve fast charging for when it provides a real service.
  • Update and repair the management system when support exists.
  • Do not store or handle a damaged battery: fire risk requires a professional channel.

Using the object also matters. Buying a battery and barely using it does not recover its manufacturing cost. Prudence seeks durability and useful service.

The numbers, with their source

The contractual threshold: 70% at 8 years

Most electric-vehicle manufacturers guarantee the battery will retain at least 70% of original capacity at 8 years or 160,000 km, whichever comes first. Under moderate climate and use conditions, NMC/NCA chemistries typically retain 82% to 88% of their capacity at that point, and LFP between 88% and 92% — both above the guaranteed minimum, confirming that the warranty figure is a contractual safety floor, not the central expectation of what actually happens in most vehicles.

Sources: MOTORWATT — EV Battery Degradation: Real Data, Rates & Fixes; Energy Solutions Intelligence — EV Battery Warranty Analysis.

Full cycles: the difference between chemistries

LFP cells typically reach 3,000 or more full cycles with annual degradation on the order of 1-1.5%. NMC/NCA cells typically deliver between 1,000 and 1,500 full cycles, with somewhat higher annual degradation, at 2-2.5%. This difference explains why LFP has become the preferred chemistry for applications where longevity matters more than maximum energy density — grid-scale stationary storage, for example — while NMC still dominates where weight and range are the priorities.

Source: Carsrave — EV Battery Guide: LFP vs NMC & Real Lifespan Facts.

What this table is missing

None of these figures are an individual guarantee: they are statistical fleet averages under moderate climate and use conditions. A vehicle that habitually operates in extremely hot climate, or that is always fast-charged to 100%, can degrade noticeably faster than these averages — this leaf's recorrido tab explains why calendar time and usage pattern matter as much as cell chemistry.

Sources