What is a self-charging hybrid?
'Self-charging hybrid' is term (used primarily by Toyota and Lexus) describing a type of hybrid vehicle that combines a petrol or diesel engine with electric power. It's also known as a 'full hybrid'.
A portion of an engine’s power – as well as energy recovered from slowing down – is used to charge a small battery, which then steps in to assist with moving the car at low speeds or when accelerating in order to improve efficiency.
However, unlike a plug-in hybrid, a self-charging hybrid can't be charged from a socket or charge point and is typically only capable of driving a mile or two on electric power alone.
Meanwhile, a 'mild hybrid' is a car with a starter generator in place of the starter motor. They enable a car to idle for longer in stop-start traffic, but they can't provide electric-only drive.
How do self-charging hybrids work?
Self-charging hybrids usually employ one or more electric motors to aid the performance of a car’s petrol or diesel 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.
This has the effect of saving fuel, therefore improving fuel economy on typical journeys through 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 will reduce CO2 emissions, making them 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 – for example, the Toyota Prius – you can choose how harshly 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 around 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. Benefit-in-Kind (BiK) bands are based on CO2 emissions, with cleaner cars attracting lower rates, usually between 9% and 17%.
Road tax (VED) is lower for self-charging hybrids, costing £130 per year instead of the £140 charged annually for conventional models. Hybrid vehicles emitting less than 75g/km can also enter the London Congestion Charge zone for free, although few self-charging hybrids can hit this mark – it's more the realm of plug-in hybrids.
You may find that self-charging hybrids can potentially be more economical than plug-in hybrid vehicles over long distances. This is because a plug-in hybrid with a bigger battery will effectively be carrying more weight that isn’t contributing to the performance of the car for most of the journey, meaning the petrol or diesel engine would have to use more fuel to cover the same distance. For example, if you were to go on holiday somewhere you couldn't charge up for the duration of your stay, the only way the battery could gain charge would be via regenerative braking.
Self-charging hybrids are also very relaxing to drive. Almost all of them have automatic gearboxes that are designed to be smooth and quiet, and light controls can reduce fatigue on the road.
Do self-charging hybrids have any drawbacks?
While self-charging hybrids are likely to be a better prospect for long-distance drivers than plug-in hybrids, it’s important to remember that they bring no efficiency benefits at motorway speeds. On fast roads, you're entirely reliant on the internal combustion engine, and if you travel cross-country a lot, then a pure petrol or diesel will 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 so. So if you frequently drive short distances and you can charge a car at home or at work, a plug-in hybrid might be a more cost-effective option. Not only will you save money by travelling on electric power rather than petrol or diesel, you’ll still have the option of driving further afield using conventional fuel should the need arise.
Finally, self-charging hybrids are unlikely to suit driving enthusiasts. Most self-charging hybrids are designed to save fuel and make driving a relaxing and pleasant experience, as opposed to an exciting or involving one. That said, there are fast hybrids out there if you really want the best of both worlds…