What is a mild hybrid car?

A boost to fuel economy without the cost of a full hybrid car: why mild hybrid technology could well be coming to your next car.

Simon Ostler
Sep 7, 2021

As the world builds towards an eco-friendly future, consumer preferences are heading in a greener direction, especially when it comes to the type of cars a growing proportion drivers are buying. Couple that with government initiatives to crack down on fossil fuel emissions, and car manufacturers have their work cut out to adjust to an evolving set of needs.

Over the past decade we have seen the motoring industry take a sharp turn towards efficiency, developing hybrid technology designed to reduce the exhaust emissions of their cars and limit their impact on the environment. There are different types of hybrid car, though, and each has a distinct approach to improving fuel efficiency: plug-in hybrids, full or self-charging hybrids, and mild hybrids.

Our guides to each diffrent type of hybrid car will help you to find out more about those, but here we will be focusing on the mild hybrid, which introduces a relatively small electric motor to a standard combustion engine to deliver a small amount of hybrid assistance under acceleration.

Mild hybrid engines are designed to remain as close to a traditional petrol or diesel as possible in terms of functionality. Whereas a plug-in hybrid will feel far removed from a standard driving experience many drivers will be used to. For this reason, a mild hybrid will not offer anything like the kind of economical benefit you'll find in a full or plug-in alternative.

But, for this reason, they are also the most affordable form of hybrid car. All of the complex and expensive tech that goes into a plug-in hybrid car understandably bumps up its purchase price, whereas the relatively minor additions involved in creating a mild hybrid setup have much less cost involved. A mild hybrid will also prove to be the most convenient hybrid when it comes to daily life, too. There are no batteries that need charging, so you can drive it and refuel it like any other petrol or diesel car.

What is a mild hybrid car?

Mild hybrids have a slightly bigger battery than normal cars. This might not sound very exciting, but the extra electrical capacity can provide enough energy to power a small electric motor that supplements the power of the engine under acceleration.

This minor assistance helps to reduce the stress on the petrol or diesel engine, thereby reducing the amount of fuel it burns and improving efficiency. This leads to better fuel economy - meaning that your fuel bills are a little lower than they would otherwise be - and you get lower CO2 emissions from the exhaust.

The best systems also allow the engine to be switched off for short periods while the car is travelling at a steady speed, further improving efficiency.

Mild hybrid car deals
Volkswagen Golf

BuyaCar prices from £22,800
Monthly finance from £339*

Ford Fiesta

BuyaCar prices from £15,495
Monthly finance from £248*

Mazda CX-30

BuyaCar prices from £19,350
Monthly finance from £267*

Mild hybrids: pros and cons


Cheapest type of hybrid car
Some fuel economy benefits
Performance improvements


Limited improvement in efficiency
More expensive than non-hybrid cars
Small number of models so far

How do mild hybrid cars work?

Mild hybrid cars usually have a 48 volt battery that can deliver more power than the standard 12 volt battery that’s under the bonnet of most cars. The example of the Kia Sportage, above, shows the battery at the back of the car.

These batteries can store more energy than a conventional car but they are much smaller than those in other types of hybrid car. A Toyota Prius, for example, can store three times as much electricity as an Audi A7 or Kia Sportage mild hybrid.

The extra capability still comes in useful to drive a small motor next to the car’s engine (labelled mild hybrid starter and generator, above). In some systems, such as those used by Kia, the battery powers the motor under acceleration, reducing the need to work the engine hard, which is particularly inefficient.

In other mild hybrids, the motor can keep the car going at a steady speed, which doesn’t require a great deal of energy, allowing the engine to be switched off - for up to 40 seconds in mild hybrid-powered Audis. During this time the car isn't burning any fuel - again reducing your fuel bills and cutting the emissions of the car.

The batteries recover energy as the car slows down and brakes, shown by the yellow arrows in the diagram above. They can also be recharged during steady driving, when the car calculates that it is efficient to do so. This ensures that the hybrid tech causes the least possible drain on the petrol or diesel engine and provides the greatest electric boost possible.

Used mild hybrid cars

Of all the hybrid options on the market, mild hybrids make up the smallest percentage at the turn of the 2020s. Due to their 'mild' nature, their impact on efficiency is much less pronounced, and while prices for mild hybrids remain higher than regular petrol or diesel alternatives, they aren't as expensive as plug-in hybrids, which feature much larger battery packs and more substantial electric motors.

Used car deals. however, see that price gap close almost completely, with just a few hundred pounds proving the difference. So if you fancy saving yourself a bit of fuel money each month, a used mild hybrid might be a convenient way to go about it. Used mild hybrids are likely to become available in ever greater numbers as this tech is integrated into ever more new cars.

Common examples of mild hybrid models available now are 'MHEV' versions of the Mazda 2 and Mazda 3, a number of Audi 'S' models, and several versions of the Kia Sportage and Hyundai Tucson. Suzuki also implements mild hybrid technology in the Swift and Ignis ranges, in models which are labelled 'SHVS'.


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