Battery degradation isn't linear, packs lose most in the early years and then settle onto a slow decline toward a floor. Liquid-cooled cars typically lose a couple of percent a year; air-cooled cars like the early Leaf lose noticeably more, especially with heavy rapid charging.
Our model uses an exponential-approach curve calibrated per chemistry and cooling type, blended from the car's age and mileage. It never pretends a pack falls off a cliff, and it never assumes zero loss either.
Set the exact age and odometer on any car page to see the estimated state of health and what it does to real range. Before buying, a battery health check confirms the actual figure for that specific car. Sources are listed on the methodology page.