Mild Hybrid

A mild hybrid, abbreviated MHEV, is a petrol or diesel car fitted with a small electric system that assists the combustion engine without ever powering the wheels on its own. It exists to capture much of the fuel-saving benefit of hybridisation at a fraction of the cost and complexity of a full hybrid, making it an easy way for manufacturers to lower fleet emissions across large numbers of otherwise conventional models. Because the changes are modest, mild-hybrid technology has spread rapidly across mainstream petrol and diesel ranges.

Most modern mild hybrids use a 48-volt electrical subsystem, separate from the car's traditional 12-volt network, paired with a small lithium-ion battery of perhaps 0.1 to 0.5 kilowatt-hours. At the heart of the system is a belt-driven or crank-mounted starter-generator that replaces the conventional alternator. This single compact machine can act as a motor, adding a brief torque boost of typically 10 to 20 kilowatts during acceleration, or as a generator, recovering energy that would otherwise be wasted. The higher 48-volt potential allows meaningful power to flow through thinner, lighter wiring than a 12-volt system could carry.

In day-to-day driving the benefits are subtle but real. The starter-generator recovers energy during braking and deceleration, storing it in the small battery, and then feeds it back to relieve the engine when pulling away or cruising. It also makes the stop-start function far smoother and more frequent, restarting the engine almost imperceptibly and even allowing the car to switch off and coast with the engine idle. The net effect is fuel and CO2 savings of roughly 5 to 15 per cent in real-world use, together with quieter, more refined low-speed manners.

Mild hybrids sit at the entry point of a spectrum. Unlike a full hybrid (HEV), which can creep and accelerate on electricity alone, or a plug-in hybrid (PHEV), which adds a large battery and an external charging socket for a meaningful electric-only range, the MHEV battery is never large enough to drive the car. Systems vary in their voltage — some budget designs use 12 or 24 volts — and in whether the motor is belt-mounted (BSG) or integrated between the engine and gearbox (ISG), the latter allowing slightly stronger assistance.

The main limitation is precisely that lack of electric-only running: a mild hybrid is fundamentally a combustion car and cannot be plugged in, so the savings, while worthwhile, are bounded. The system is largely maintenance-free for the owner, as the small battery is managed automatically and is not expected to need replacement within the car's normal life. In essence, the mild hybrid is the lightest-touch member of the electrified family, bridging conventional petrol engines and the more capable HEV and PHEV designs that share its principle of recovering and reusing braking energy.