What is a self-charging hybrid?

The term 'self-charging hybrid' is appearing increasingly frequently, but what exactly does it mean? Here, we outline the upsides and downsides of this technology

The term "self-charging hybrid" is marketing parlance (used mainly by Toyota and Lexus, but also Kia) to describe a type of hybrid car that combines a petrol or diesel engine with a small electric motor and battery. It's also known as a 'full hybrid'.

A portion of the petrol or diesel engine’s power – as well as energy recovered from braking – is used to charge the battery, which sends power to a small motor that helps propel the car at low speeds or when accelerating, in order to improve efficiency.

In contrast to a plug-in hybrid, a self-charging hybrid can't be charged from a power socket or charging point and is typically only capable of driving a mile or two on electric power alone.

There are also mild hybrids – cars with a starter generator instead of the traditional starter motor. These can idle without the engine for longer in stop-start traffic, and sometimes also allow 'coasting' when cruising gently. But they can't provide electric-only driving.

How do self-charging hybrids work?

Self-charging hybrids usually have one or more electric motors, which are used to boost the performance or efficiency of a car’s petrol or diesel engine. Nearly all self-charging hybrid models use a petrol engine.

Once the battery has acquired enough charge, a self-charging hybrid can use this additional energy to help the car gain speed, reducing the burden on the internal-combustion engine in what is usually one of the most fuel-hungry parts of the driving process. This saves fuel, therefore improving fuel economy during typical trips within towns and cities.

Most self-charging hybrids are also capable of moving under electric power alone for short distances, which is useful in slow-moving traffic and during manoeuvres like parallel parking.

As well as conserving fuel, self-charging hybrids tend to have lower CO2 emissions than their purely petrol counterparts, making them somewhat better for the environment. Self-charging hybrids are so-named because you can't charge the battery externally: all the energy is harvested from either the engine, the brakes, or merely the act of slowing down.

In some self-charging hybrids – the Toyota Prius, for example – you can choose how aggressively a car will decelerate the moment you take your foot off the accelerator. The more severe the setting, the more energy you will recover to store in the battery. In some cases, this will allow you to do some of your driving without using the brake pedal at all – although you’ll still need to use it if you need to slow down suddenly or perform an emergency stop.

What are the benefits of a self-charging hybrid?

If you frequently drive in built-up areas or busy towns, self-charging hybrid technology will reduce the workload on a petrol or diesel engine, reducing your running costs and curbing air pollution, too.

Self-charging hybrids usually emit less CO2 than their non-hybrid equivalents, which makes them more affordable as company cars. A car's Benefit-in-Kind (BiK) band is based on its CO2 emissions, with cleaner cars attracting lower rates.

Road tax (VED) is slightly lower for self-charging hybrids, at £140 a year instead of the £150 annual fee for conventional models. Hybrid vehicles emitting less than 75g/km can also enter the London Congestion Charge zone for free until October 2021, although almost no self-charging hybrids can hit this mark – it's more the realm of plug-in hybrids, with a longer all-electric range.

You may find that a self-charging hybrid could cost less to run than a plug-in hybrid, depending on how you use it. If you aren't able to charge a plug-in hybrid regularly, the battery effectively becomes dead weight, making a self-charging hybrid a better choice for some drivers.

Do self-charging hybrids have any drawbacks?

While a self-charging hybrid is likely to be a better prospect for some drivers than a plug-in hybrid, it’s important to remember that their efficiency benefits are much reduced at motorway speeds. On fast roads, you're mostly reliant on the internal combustion engine, and if you travel cross-country a lot, then a pure petrol, diesel or electric car will probably be more suitable.

Because of their small batteries, self-charging hybrids can’t travel very far on electric power alone; usually no more than a mile or two. So if you frequently drive short distances and you can charge at home or at work, a plug-in hybrid might be more cost-effective. Not only will you save money by using electric power rather than petrol or diesel, you’ll still have the option of driving further afield using conventional fuel should the need arise.

It's also worth noting that while there are a few four-wheel drive SUVs available with the technology on board, these can't match their diesel equivalents for pulling power. A four-wheel drive Toyota RAV4, for example, has a maximum braked capacity of 1,650kg, while most similarly sized diesel SUVs will manage around 2,000kg to 2,500kg. It's worth considering this if you're a caravan or trailer user.

Finally, self-charging hybrids are unlikely to suit driving enthusiasts. Most of them are designed to save fuel and make driving a relaxing and pleasant experience, as opposed to an exciting or involving one. However, there are some fast hybrids out there if you really want the best of both worlds.