What is MHEV: Guide to Mild Hybrid Electric Vehicles — Lectron EV Skip to content
What is MHEV: Guide to Mild Hybrid Electric Vehicles

What is MHEV: Guide to Mild Hybrid Electric Vehicles

Mild Hybrid Electric Vehicles (MHEVs) have evolved as a safe halfway point between fully electric vehicles and conventional gasoline-powered cars. While not entirely an eco-hero, MHEVs provide a balanced solution, with slightly improved fuel efficiency as compared to traditional Internal Combustion Engine (ICE) vehicles.

What is a Mild Hybrid Car?

The BMW M340i is one of the most popular mild hybrids in the market.
Image courtesy of BMW Blog

A Mild Hybrid Electric Vehicle, also known as a "mild hybrid," is a type of hybrid vehicle that combines a conventional internal combustion engine (ICE) with a small electric motor and a battery pack. Unlike full hybrid or plug-in hybrid vehicles, MHEVs cannot operate solely on electric power. Instead, they use the electric motor to assist the engine during acceleration, deceleration, and when cruising at low speeds. The primary purpose of the electric motor is to improve fuel efficiency and reduce emissions.

How does an MHEV work?

A representation of the Hyundai 48-Volt Mild Hybrid System
Photo Courtesy of Hyundai

A Mild Hybrid Vehicle operates by combining a conventional gasoline or diesel engine with a small electric motor and battery system. In a nutshell, mild hybrid technology uses electricity to support the engine and recover energy during braking, which leads to better fuel efficiency and reduced emissions compared to traditional vehicles.

Here's a breakdown of how the mild hybrid system works:

  1. Electric assistance: MHEVs have a small electric motor and a small battery, typically around 48 volts. This electric motor assists the gasoline or diesel engine during various driving situations.

  2. Energy recovery: When you brake or slow down, the electric motor acts like a mini-generator, capturing and storing some of the kinetic energy that would otherwise be lost as heat during braking. This is called regenerative braking. This energy is stored in the battery for later use.

  3. Start-stop function: Many MHEVs come equipped with a start-stop system. It automatically shuts off the engine when the vehicle comes to a stop (e.g., at a red light). The electric motor quickly restarts the engine when you release the brake.

  4. Power boost: During acceleration and when extra power is needed, the electric motor provides a boost of power to assist the engine. This helps improve fuel efficiency and reduce emissions.

  5. Limited electric-only mode: Some MHEVs can briefly operate on electric power alone, but only at very low speeds or under light loads. It's not a full electric mode like in plug-in hybrids.

What are the advantages and disadvantages of a mild hybrid?

Mild Hybrid Vehicles come with their fair share of advantages and disadvantages. These factors might help you decide whether MHEVs align with your driving needs and preferences.


  1. Improved fuel efficiency: MHEVs assist the internal combustion engine, reducing its workload. This results in better fuel economy, saving you money at the pump.

  2. Reduced emissions: With the electric motor's assistance, MHEVs emit fewer greenhouse gases and pollutants compared to traditional vehicles, contributing to cleaner air and reduced environmental impact.

  3. Smooth start-stop: MHEVs often feature a start-stop system that shuts off the engine when stopped (e.g., at traffic lights). The electric motor restarts the engine seamlessly, enhancing comfort and fuel savings in city driving.

  4. Regenerative braking: The regenerative braking system captures and stores energy during braking, which would otherwise be wasted as heat. This energy is reused, further improving efficiency.

  5. Transition to green: MHEVs serve as a bridge between conventional vehicles and full hybrid or electric cars. They allow drivers to make a greener choice without fully committing to electric driving.

  6. Resale value: MHEVs typically have better resale value than traditional vehicles due to their improved fuel efficiency and lower emissions.


  1. Limited electric-only range: Unlike plug-in hybrids or electric vehicles, MHEVs cannot operate solely on electric power for extended distances. Their electric-only mode is limited in duration and range.

  2. Modest fuel savings: While MHEVs improve fuel consumption, the savings at the pump may not be as substantial as with full hybrids or electric cars, which can deter some buyers.

  3. Higher initial cost: MHEV technology adds to the upfront cost of the vehicle. Depending on your driving habits, it may take several years to recoup the extra investment through fuel savings.

  4. Less environmental impact: While MHEVs are greener than traditional vehicles, they may not achieve the same level of emissions reduction as full hybrids, plug-in hybrids, or electric cars.

  5. Charging not required: Some consumers prefer vehicles that can be charged externally, like plug-in hybrids or electric cars, which may offer longer electric-only ranges.

  6. Limited all-electric performance: MHEVs can provide an electric boost but are not designed for high-speed or long-distance electric-only driving, limiting their electric performance.

What’s the difference between a mild and full hybrid car?

Image courtesy of Quora

Mild Hybrid Electric Vehicles (MHEVs) and Full Hybrid Electric Vehicles (HEVs) share the hybrid label, but they operate quite differently. Here's a detailed comparison:

  1. Electric power:

    • MHEV: MHEVs use a small electric motor mainly to assist the gasoline or diesel engine. They can't operate solely on electric power.

    • HEV: HEVs have a more substantial electric motor and can operate on electric power alone for short distances at low speeds.

  2. Battery size:

    • MHEV: MHEVs have a smaller battery, often around 48 volts.

    • HEV: HEVs have a larger battery, usually over 200 volts, which allows for more electric-only driving.

  3. Charging:

    • MHEV: MHEVs don't require external charging. Their batteries are charged through regenerative braking and the engine.

    • HEV: HEVs also don't need to be plugged in. Their batteries are charged through regenerative braking and the engine, just like MHEVs.

  4. Electric-only mode:

    • MHEV: MHEVs can't operate on electric power alone, except for brief moments at very low speeds.

    • HEV: HEVs can operate on electric power alone for short distances and at low speeds, enhancing fuel efficiency in stop-and-go traffic.

  5. Fuel efficiency:

    • MHEV: MHEVs offer some fuel efficiency improvements compared to traditional vehicles.

    • HEV: HEVs typically provide more significant fuel savings due to their ability to operate on electric power.

  6. Cost:

    • MHEV: MHEVs are usually less expensive than HEVs because they have smaller batteries and less complex electric systems.

    • HEV: HEVs are generally pricier due to their larger batteries and more robust electric components.

Are mild hybrids worth it?

If you prefer the familiarity of a conventional combustion-engine car but seek a boost in efficiency, a mild hybrid could be the ideal choice. It offers a slight uptick in fuel economy without veering too far from the conventional vehicle experience.

On the other hand, if you're aiming for frequent electric-only driving, it's worth considering a conventional hybrid or plug-in hybrid as a more suitable transition on your way to a fully electric vehicle. It's worth noting that these alternatives often come with a higher price tag compared to mild hybrids, so while they offer enhanced electric capabilities, they can be a bit heavier on the initial investment.


  • What is the difference between MHEV and PHEV?

    MHEVs use a small electric motor to assist the engine and don't require external charging. PHEVs have larger batteries that can be charged externally and can operate on electric power alone for longer distances and at higher speeds.

  • What are the benefits of an MHEV?

    MHEVs offer improved fuel efficiency, reduced emissions, smoother start-stop functionality, regenerative braking, and a lower carbon footprint compared to traditional vehicles.

  • Are mild hybrid cars worth buying?

    Whether MHEVs are worth buying depends on your driving patterns, budget, and environmental concerns. They can be a good choice for city driving and short trips, but the cost savings may take time to offset the higher initial price.

  • Do mild hybrids need charging?

    No, mild hybrids do not require external charging. They recharge their small batteries through regenerative braking and the engine, so no plug-in charging is needed.

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