All batteries will degrade from the first day of use, but with hybrid vehicles the impact is not as significant as the batteries are not in use all of the time and are only partly responsible for providing drive to the wheels.

Although mileage plays a part in how much batteries have degraded it also depends on how the vehicle has been driven. Urban driving will mean more use of the batteries compared to motorway speeds, so theoretically a car that spends more time on the motorway will have a longer battery life. A degraded battery will be able to operate less of the time, so fuel economy will be reduced as the battery ages.

However, cars like the Toyota Prius have been around a long time and many examples have gone past 200,000 miles, so a hybrid is capable of big mileages.