A range extender (REX), or range extended electric vehicle (REEV), is a battery-powered electric car with an on-board range extender generator called an auxiliary power unit (APU). When the battery charge is low, the range extender starts automatically and charges it up. Unlike self-charging and plug-in hybrids, the range extender doesn’t power the car’s wheels directly, it only charges the battery.

What’s the advantage of a range extender? 

In theory, the main advantage of a range extended electric car is that range anxiety should become a thing of the past because the battery will always be topped up by the range extender before it goes flat. As long as you don’t run the fuel tank dry, you won’t get stuck or have to worry about the availability of a public charger.

What are the disadvantages of a range extender? 

The main disadvantage is complexity and cost. The range extender engine needs the same sort of care as a conventional car or hybrid, such as routine services, including oil changes. REEVs are also more expensive than a pure electric version of the same car. You still have to visit a filling station forecourt from time to time to put fuel in the tank, something you never have to do with a battery-only EV.

Is a range extended vehicle as efficient as a battery-only EV? 

Range extenders and the fuel that goes with them add extra weight which saps energy from the battery when running in pure electric mode. When the engine is turned off (which should be most of the time), it’s like carrying the weight of excess baggage around. In common with conventional cars and hybrids, the engine generates emissions and CO2 when it’s running, which you may think defeats the object of owning an EV.

The BMW i3 REX was discontinued when the pure EV's battery range was extended

Is a range extended electric car the same as a hybrid? 

Forgetting whether a hybrid is a plug-in or the original self-charging hybrid for a moment, hybrids come in two main types, parallel hybrid or series hybrid. Most hybrids (the original Toyota Prius being an example) are parallel hybrids which simply means the combustion engine and electric motor can both drive the road wheels. Either the engine or the electric motor can drive the wheels or the two can combine to both feed power to the wheels at the same time.

With a series hybrid things happen one after the other. The range extender charges the battery, and the battery provides electricity to power the electric motor which drives the wheels. The engine never drives the wheels directly.

It gets a little more complicated with cars like the Nissan Qashqai e-POWER, which have no direct link between the engine and the wheels through a gearbox, so are range extenders according to the 'rules' But they have a small battery so there is not much range to extend.

Do range extender vehicles work? Should I buy one? 

In the early days of EVs when battery technology was in its infancy, EV range was generally shorter and there were fewer hybrids around, range-extended EVs seemed like a good idea. As EV battery technology has improved and range has increased, along with improvements in the charging network and increased numbers of hybrids on sale, the need for range extended electric vehicles has pretty much disappeared.

Where can I buy a range extended electric vehicle? 

The only range-extended EV you can buy new in the UK today is the 2023 Mazda MX-30 R-EV which starts from £31,250. Examples of used REEVs which are no longer on sale new in the UK are the BMW i3 REX and the Vauxhall Ampera E-REV. Used BMWs start from around the £10,000 mark and the Vauxhall from around £8,000.

The Mazda MX-30 R-EV is the only range extender currently available