What “Battery Degradation” Actually Measures

Battery degradation measures the loss of a battery’s usable energy capacity over time, expressed as a percentage of what it could originally hold — not how “old” a battery feels or how it performs on any single day. Understanding what the number represents helps make sense of the specs manufacturers publish.

What the percentage represents

If a battery is rated at a certain capacity when new and later tests show it can only hold a smaller share of that, the difference is the degradation figure. It’s a measure of maximum capacity, not of day-to-day performance, which is why a car with some degradation can still feel completely normal to drive.

Degradation isn’t linear. It tends to happen somewhat faster early in a battery’s life and then level off, rather than declining at a constant rate for the whole time you own the car. Two batteries with the same degradation percentage can also behave differently depending on the chemistry and how the battery management system handles aging cells, so the raw number alone doesn’t tell you everything about how the car will drive. This is also why two owners of the identical model and model year can report very different real-world range even at the same degradation percentage — driving style, climate, and charging habits all layer on top of the baseline capacity number in ways a single percentage can’t capture.

At a chemical level, the underlying capacity loss happens because a battery’s internal materials gradually lose their ability to hold and release charge efficiently — a process accelerated by heat, high state-of-charge storage, and frequent fast charging. This is the same basic mechanism regardless of chemistry, just at different rates depending on the technology. Some manufacturers now report degradation using a rolling average across recent trips rather than a single instantaneous reading, specifically because a one-off measurement taken right after a very hot or very cold drive can look artificially worse than the pack’s actual underlying condition. Manufacturers typically measure this through controlled charge-discharge cycling in a lab before a vehicle ever ships, then validate it against real-world fleet data as cars age on the road, which is how warranty thresholds get set with any confidence. See our piece on what degrades an EV battery over 100,000 miles for more on those specific causes, and our guide to lithium-ion vs solid-state batteries for how chemistry factors in.

In practice, this shows up as a slightly lower maximum range than when the car was new, warranties that guarantee a minimum capacity threshold over a set period, and state-of-health readings where available — a more direct measure than range alone. For more technical detail, see the Department of Energy’s Alternative Fuels Data Center.

Degradation measures maximum capacity, not how the car drives today.