Battery degradation is the main anxiety people express about electric cars, and it is difficult to find first-hand numbers over a meaningful period.
Mine has covered a reasonable distance over four years and I have measured capacity periodically. Here is what happened.
The shape of the curve
The most useful finding and the one that surprised me.
The loss was not linear. There was a relatively quick initial decline in the first year, of a few percent, then a much slower decline afterwards.
This matches what is widely reported in fleet data and in manufacturer information. Early loss is expected and is not an indication of a problem, which is worth knowing because it alarms people who measure it early and extrapolate.
After four years the total loss was modest — in the region where the car remains entirely usable for everything I bought it for, with a slightly shorter winter range than when new.
How I measured it
Worth describing because the car's own estimate is not reliable.
The range figure a car displays is a prediction based on recent consumption, and it varies with driving and temperature. It is not a capacity measurement and should not be treated as one.
What is more reliable is a full charge to a full discharge under controlled conditions, measuring energy consumed, which is impractical and hard on the battery.
The practical method is diagnostic software that reads the battery management system's own capacity estimate, which is what I used. It is an estimate too, and it is measuring the thing you want to know rather than a proxy.
Comparing readings from the same tool over time is more meaningful than any single reading.
What appears to accelerate it
From the research literature and from what fleet data suggests, rather than from my sample of one.
Heat, consistently identified as the largest factor. Batteries kept hot degrade faster, which is why vehicles in hot climates show worse outcomes and why thermal management systems matter.
Time spent at high state of charge. Sitting at full for extended periods is harder on cells than sitting at a middle charge.
Frequent rapid charging, which generates heat and stresses cells more than slow charging, though the effect in well-managed vehicles appears smaller than early concerns suggested.
And deep discharges to very low states of charge.
What I changed as a result
Charging to eighty percent for daily use, and to full only before a long journey. Most vehicles have a setting for this and it is one of the few user-controllable factors.
Avoiding leaving it at full or very low for days, particularly in hot weather.
Using slow charging where time allows and rapid charging where it does not, which is what most people do anyway.
And parking in shade where practical in summer, which sounds marginal and is consistent with heat being the dominant factor.
The warranty position
Worth knowing because it defines the actual risk.
Battery warranties commonly cover a period and a distance, with a capacity threshold below which the battery is considered defective. That threshold is typically around seventy percent of original capacity.
Which means degradation above that level is considered normal wear and is not covered, and the warranty protects against premature failure rather than against gradual loss.
Reading the specific terms matters, since the threshold, the measurement method and what constitutes a valid claim vary between manufacturers.
The second-hand implication
Where this actually matters commercially.
For a used electric car, battery health is the single most important thing about it and it is not visible in the way mileage is.
A capacity report from diagnostic software should be treated as essential rather than optional when buying, in the way a service history is for a petrol car.
Some manufacturers provide certified health reports. Independent testing services exist in most markets.
Buying without one is buying blind on the most expensive component in the vehicle, and the spread between well-treated and poorly-treated examples of the same model and age is substantial.
What I would say to somebody worried about it
The evidence from real fleets is considerably more reassuring than the anxiety suggests. Typical degradation over a normal ownership period leaves a usable car.
The failure mode people fear — a battery suddenly needing replacement — is rare in modern vehicles and is what the warranty exists for.
The realistic concern is a gradually shorter range, which matters if you bought a car with barely enough range for your needs and does not if you had margin.
Second life and recycling
Where these batteries go, since it comes up whenever degradation does.
A battery below the threshold for vehicle use frequently retains substantial capacity, which is adequate for stationary storage, and second-life applications for grid and domestic storage are established.
Recycling processes for lithium chemistries have improved considerably and recovery rates for the valuable metals are high in properly equipped facilities.
The constraint has been volume, since relatively few vehicle batteries have reached end of life yet, which means the infrastructure is being built ahead of the wave rather than in response to it.