What a 70 per cent warranty floor actually commits a maker to

Two mechanisms, and conflating them is why the question "how long does a battery last" has no single answer.

Calendar ageing happens whether you drive or not

Chemical side reactions inside the cell continue while the car is parked. They consume lithium that is then no longer available to carry charge, and they run faster when the cell is hot and when it is held at a high state of charge.

This is the practical reason makers advise against leaving a car at 100 per cent for long periods, and why a daily charge limit of 80 per cent is a common recommendation for cells that are not lithium iron phosphate. A pack parked full in a hot climate ages measurably faster than the same pack parked half full in a cool one.

Cycle ageing happens when you use it

Every full charge and discharge does a small amount of mechanical and chemical damage. It is counted in full equivalent cycles, so two charges from 50 to 100 per cent count as one cycle, not two. Depth matters: shallow cycles in the middle of the range are gentler than deep ones that run the pack to both extremes.

Chemistry matters here more than anything the driver does. Lithium iron phosphate cells are generally rated for more cycles than nickel manganese cobalt cells, which is a large part of why they took over the standard-range end of the market even though they store less energy per kilogram. The cell, module and pack structure is what that density figure refers to.

Rapid charging adds heat and stress, and a pack fast charged exclusively will degrade faster than one charged slowly. The effect is real and it is smaller than most people assume, because thermal management systems exist precisely to contain it.

What the warranty actually says

The common European and American terms are eight years or about 160,000 km, with the maker undertaking to repair or replace the pack if state of health falls below a stated threshold, usually around 70 per cent. California requires longer terms than the US federal minimum.

Read that as a floor, not a forecast. It is the point at which the maker accepts liability, set deliberately below what they expect a normal pack to do. A pack that lost 30 per cent in eight years would be a poor one; the warranty exists for the poor ones.

The number that is genuinely hard to get is what a typical pack does, because the honest answer requires fleet data over a decade and most models have not existed that long. Treat any single figure for "average degradation" with suspicion, and check what fleet it came from and over what period.

Sources

1. US Department of Energy, Alternative Fuels Data Center: electric vehicle batteries, read 17 September 2026
2. California Air Resources Board, Advanced Clean Cars II durability requirements, read 17 September 2026