What is a range extender and how do they work?

You will likely have come across the terms electric car, hybrid car and plug-in hybrid car, and even a hydrogen fuel-cell car, but the term 'range extender' is perhaps less well known.

This refers to a kind of electric car with both an electric drivetrain (comprised of one or more electric motors and a battery) and an internal-combustion engine. While that's the same basic setup you'll find in a hybrid, the key difference is that in a range extender, the internal-combustion engine functions only as a small generator for the batteries – it never drives the car's wheels directly.

Unlike a plug-in hybrid (which usually has distinct electric-only, engine-only and combined electric-plus-engine driving modes) or a regular hybrid (which shuffles between the two power sources automatically), a range extender is only ever driven by the electric motor, so it always feels the same from behind the wheel, albeit with a little bit of extra noise if the engine is charging the battery.

Range extenders are not very common, and look to be headed for obsolescence as battery technology and charging infrastructure develop, eliminating the 'range anxiety' that was sometimes associated with early electric cars. In order to work properly, they need to be both refuelled at a petrol station and recharged at a home or public charging point.

The BMW i3 was initially offered in both range-extender and pure-electric forms, but the latter has been the only choice for some time now. Another range extender that was sold in the UK was the Chevrolet Volt/Vauxhall Ampera, but both versions of this car have now been discontinued.

The big selling point behind range extenders was that they effectively erased range anxiety. This is because the petrol engine not only significantly boosts the vehicle’s range, but also because it can be topped up at regular fuel stations without the need to plug the vehicle in to charge.

However, a downside to range extenders is that (unlike pure-electric models) they're not exempt from road tax under the current Vehicle Excise Duty rates. This is because with the engine on board, they emit some CO2, resulting in an annual £135 bill.