What is a mild hybrid car?

Updated: 21 August 2023

You might have heard a car being described as a ‘mild hybrid’, but what does that actually mean? How is it different from other types of hybrid cars? And do you have to plug it in? Read on to find out.

A mild hybrid car (also known as a mild hybrid electric vehicle, or MHEV) has a petrol or diesel combustion engine and a small battery-powered electric motor that helps improve your fuel economy while reducing your car’s carbon dioxide (CO2) emissions.

Mild hybrids are the simplest form of hybrid car. They’re different from conventional hybrids (often known as full hybrids or ‘self-charging’ hybrids) and plug-in hybrids because the electric motor doesn’t drive the wheels directly. 

Different car manufacturers have mild hybrid systems that work in slightly different ways, but they all follow this broad principle. Because mild hybrid cars are simpler than other hybrid systems, they usually cost less to buy.

The electric motor in a mild hybrid car is a battery-powered ‘starter/generator’ that replaces the starter motor and alternator you’d traditionally find in a petrol or diesel car.

In a conventional car, the starter motor starts the car, while the alternator turns mechanical energy into electrical power to recharge the battery and power the car’s electrical components. 

In a mild hybrid the starter/generator does all of this and, without getting too technical, it’s also connected to the car’s engine or transmission so it can help out the engine when you accelerate. This means that the petrol or diesel engine has less work to do, so it uses less fuel.

Most mild hybrid cars have a 48-volt battery as well as the 12-volt battery you get with conventional petrol or diesel cars. The starter/generator harvests the energy created when you use the brakes and stores it in the battery so it can be used when needed.

All hybrid cars use battery-powered electrical systems to give better fuel economy than if they only had engines. A conventional full hybrid has an electric motor connected directly to the wheel, which in most cases means the car can be driven on electricity alone for short distances, with no exhaust emissions.

But a mild hybrid’s electrical system isn’t connected to the wheels, so you can’t drive it solely on electric power. You can read more about the differences between mild hybrids, self-charging hybrids and plug-in hybrids here.

The batteries that power mild hybrid systems are recharged by ‘regenerative’ braking. That means that when you press the brake pedal, or even just lift off the throttle, the starter-generator reverses the direction of its spin and generates electricity, which is fed back to the batteries.

You don’t plug a mild hybrid into a power outlet to recharge its batteries. Only plug-in hybrids and electric cars are recharged that way.

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Driving a mild hybrid feels much the same as driving a ‘normal’ car, but there are some small differences. Most modern cars have a stop/start system, which shuts down the engine when you’re stationary to save fuel. But in a mild hybrid, this function is taken care of by its starter/generator, which usually means you get less of a jolt when the engine starts – you may not even notice it.

The regenerative braking that recharges the battery can affect how the brakes feel, and the car may slow down more than you expect when you press the brake or lift off the accelerator.

Some mild hybrid systems are powerful enough to give a boost to the engine’s acceleration, but you’ll probably only notice the difference if you drive a mild-hybrid car immediately after driving a regular model.

There’s no hard and fast rule on the fuel economy you can expect from a mild-hybrid car, but it should be better than a car with a conventional petrol or diesel engine.

Other than that, the normal principles apply. A large, heavy car with a powerful engine will get through more fuel than a small, light car with less power, regardless of whether it’s a mild hybrid.

Although mild-hybrid systems reduce your car’s fuel consumption and CO2 emissions, the reductions aren’t as large as you get with a conventional hybrid or a plug-in hybrid. Mild hybrid cars also don’t give you the option of zero-emission, electric-only power, which you get with all plug-in hybrids and most full hybrids.

Some mild hybrid models cost a bit more than an equivalent non-mild hybrid version, but the technology is fast becoming the norm for new cars.

In most cases, mild hybrids give you better fuel economy and emit less CO2, which should lower the amount of Vehicle Excise Duty (car tax) you’ll have to pay. The engine usually feels smoother and more responsive, which makes driving easier and more enjoyable. The systems are also far less complex than other hybrid technologies – they don’t need to be plugged in and they feel identical to conventional cars while driving, which is great if you want familiarity behind the wheel.

While the fuel-saving benefits of mild hybrid technology are relatively small compared with other types of hybrid cars, the cost of mild hybrid cars is far lower.

The chances are that, especially with used cars, the price difference between a conventional model and one equipped with mild hybrid tech will be minimal. With that in mind, we can’t see any reason not to go for a mild hybrid car. That said, if big fuel savings are near the top of your priority list, another type of hybrid or a full-electric car could be a better option.

Most car brands already have several mild hybrid models in their ranges. For example, every new version of the latest BMW 5 Series that isn’t a plug-in hybrid is a mild hybrid, while almost all new cars from Volvo are either mild hybrids, plug-in hybrids or full-electric cars. Every new Fiat 500 is a mild hybrid too, although Fiat badges the car simply as a 'hybrid'.

Within the next few years, pretty much every car that isn’t a self-charging, a plug-in hybrid, or fully electric will have to be a mild hybrid to meet the latest emissions standards.

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