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What is a self-charging hybrid?

The term 'self-charging hybrid' appears frequently in the advertising and marketing of certain models, but what exactly does it mean? Here, we outline the upsides and downsides of this technology

Lexus hybrid system screen

The term 'self-charging hybrid' is employed principally by Japanese brands Lexus and Toyota, as well as the South Korean company Kia, to describe a type of hybrid car that uses for propulsion a diesel or petrol engine combined with a small battery feeding a compact electric motor. Another term used to describe this kind of setup is 'full hybrid'.

Some amount of the combustion engine's power – in combination with energy captured from the brakes when you slow down – is recovered and used to top that small battery. The battery then transfers power to the electric motor, which can move the car on its own while parking or in slow traffic. The motor can also give an extra boost to the petrol engine when accelerating; in both scenarios, less fuel is used than would otherwise be the case.

Unlike a plug-in hybrid, a self-charging hybrid like this cannot be recharged from a domestic socket or a public charging point. Normally they can only drive for a mile or two at most on pure-electric power, but usually the idea is that short periods of electric running throughout a journey can add up to significant fuel savings overall.

Self-charging hybrids should not be confused with mild hybrids. These cars with a starter generator instead of the usual starter motor. They're generally capable of idling without the engine on for longer in stop-start traffic, and in some cases can allow 'coasting' when cruising gently. But they're not capable of electric-only driving for any distance.

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 charged up sufficiently, a self-charging hybrid can use this additional energy to help the car gain speed, reducing the burden on the internal-combustion engine. 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 at lower speeds – most 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 via a cable: 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 drive 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 reducing 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 £145 a year instead of the £155 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 regularly, the battery effectively becomes dead weight, making a self-charging hybrid a better choice for some drivers.

Do self-charging hybrids have 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; if you travel cross-country a lot, then a pure petrol, diesel or longer-range electric car will probably be more suitable.

Because of their small batteries, self-charging hybrids can’t travel 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 relaxing and pleasant, as opposed to exciting or involving. However, there are some fast hybrids out there if you really want the best of both worlds.

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