The Mild Hybrid Vehicle Explained: A Comprehensive Guide

What is a mild hybrid car?

A mild hybrid utilises a battery-powered electric motor to support a petrol or diesel engine. This means that mild hybrids offer improved efficiency and lower emissions compared to their traditionally-fuelled counterparts.

What is the difference between a mild hybrid car a standard hybrid car?

There are three main hybrid vehicle types, MHEVs, hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs).

Here’s how they differ:

• A MHEV blends a traditional fuel engine and electric power, meaning it does not have to be plugged in. Instead, MHEVs have a small electric motor, which is attached to a petrol or diesel engine. The small electric motor provides a boost when accelerating.
• Much like MHEVs, HEVs don’t need external charging, but they have a larger battery, which allows them to make short trips on electric power alone, providing greater fuel savings.
• A plug-in hybrid vehicle (PHEV) often has a smaller fuel engine than other hybrids and can be powered exclusively by a large battery pack. Unlike MHEVs and HEVs, PHEVs require external charging to make full use of their electric capabilities.

Are all mild hybrids the same?

No, not all mild hybrids are the same, but they all share a common principle: combining a conventional petrol or diesel engine with a small electric motor and battery that provide assistance during driving.

Unlike full hybrids, mild hybrids cannot run on electric power alone. Instead, the motor and battery work together, supporting the engine during acceleration, improving performance when stopping and starting, and boosting fuel efficiency.

There’s a wide range of mild hybrids on the market. Some have more advanced technology than others, with each designed for different purposes:

• The Audi Q8 has a feature which improves economy by working with the car’s cruise control system (meaning that it slows down when approaching corners and roundabouts, allowing it to utilise the energy collected from regenerative braking).
• The four- and six-cylinder versions of the BMW 5 Series MHEV include a 48V starter generator and second battery, which boosts torque during acceleration, and makes stop/start driving and coasting smoother.
• The Ford Focus 1.0 EcoBoost Mild Hybrid has a 48V battery that harnesses braking energy to deliver low-rev torque assist, which cuts turbo lag and improves responsiveness.

What are the other types of hybrids?

Other hybrid variants on the market include:

Parallel hybrids

A parallel hybrid car’s wheels can be powered by the engine, the electric motor, or both power sources.

At speeds of up to 15mph, only the electric motor is used for power, which makes parallel hybrids particularly efficient for city driving.

The petrol engine cuts in when the speed increases (or under hard acceleration). When you decelerate or use the brakes, the regenerative braking system garners electricity, storing it in the battery for later use. However, parallel hybrid cars can only run on electricity for up to 1.25 miles.

Range-extended hybrids

Range-extended hybrid cars differ from regular hybrids as their combustion engines act mainly as generators, producing electricity to power the electric motors driving the wheels.

PHEVs

As their name suggests, PHEVs can be plugged into an electrical outlet to recharge their batteries.

Although PHEVs have conventional engines, they also have larger batteries than the average mild hybrid. What’s more, unlike MHEVs, they can travel on electric power alone, which reduces running costs when they are charged regularly.

PHEVs can travel solely on electric power much longer than any HEV. For example, the Omada 9 SUV, boasts an electric range of 93 miles.

Mild hybrids vs other hybrid types: How do they compare?

How does a mild hybrid system work?

Mild hybrids utilise both an electric motor and a 48-volt battery with a petrol or diesel engine. The electric motor integrates with the engine to use stored energy, which provides extra torque.

The system has the capability to monitor the usage of the vehicle and therefore knows when to charge the battery - or alternatively, use it.

Mild hybrids utilise regenerative braking to charge the battery when in use. This energy is stored in the 48-volt battery, ready for use.

Is it worth buying a mild hybrid car?

Depending on your priorities, a mild hybrid car could be a great choice:

• MHEVs reduce engine load, providing a smoother ride.
• The battery in an MHEV can provide greater torque due to smoother acceleration.
• MHEVs produce less emissions than equivalent pure petrol or diesel vehicles and are cheaper to run.
• MHEVs typically retail for less than PHEVs and EVs.
• MHEVs tend to be lighter than PHEVs.
• You won’t have range anxiety thanks to the backup petrol or diesel engine.

However, there are also several potential drawbacks to consider:

• MHEVs cannot run on electric power alone - and therefore aren’t as efficient as PHEVs and EVs.
• There are no Government Grants available for MHEVs (unlike EVs).
• MHEVs still produce CO2, which may deter eco-conscious drivers.
• MHEV drivers will have to pay daily fees if they travel in London’s Congestion Charge Zone. However, the exemption for EVs will expire on December 24th 2025.

Mild hybrids and car tax: Are they cheaper to tax?

While there are no specific tax brackets for MHEVs, in some circumstances, they will be taxed according to their CO2 emission level:

• In the first year, new MHEVs will pay a ‘showroom tax’ based on their CO2 emission level (g/km).
• From the second year onwards, all cars registered from 1st April 2017, including MHEVs, will be taxed at the standard rate.
• All cars (including MHEVs) with a list price over £40,000 will pay an additional annual ‘expensive car supplement’ between years two and six.
• If you have a company car, you may pay less benefit-in-kind (BiK) tax for a PHEV than many petrol and diesel models.
• However, your specific BiK tax band will depend on your car’s CO2 emissions.
• EVs pay the least BiK tax of all, because they are classed as zero emission vehicles.

Cars may be taxed at different rates due to their CO2 emissions, engine size, and RDE2 compliance (diesel only).

Visit our car tax guide to learn more.

How eco-friendly are mild hybrid cars?

Whilst mild hybrids offer better fuel efficiency than most petrol and diesel vehicles, they are often less efficient than more advanced hybrids:

• For instance, the Ford Puma 1.0 Ecoboost 125 MHEV’s CO2 emissions figure of 123g/km compares favourably with most standard small SUVs.
• However, it is still undeniably higher than the PHEV Mini Countryman's 40g/km.
• EVs do not produce CO2 emissions at all.

So, whilst MHEVs are reasonably efficient, they don’t come close to PHEVs or EVs.

Frequently Asked Questions