What Is a Mild Hybrid (MHEV) Car?

A mild hybrid or MHEV is a vehicle that uses the same basic tools as other types of hybrids — electric motors and batteries — but they use them to assist the internal combustion engine rather than partially supplant it. Mild hybrid cars cannot power the wheels on their own and they drive much like a traditional gas-powered vehicle.

After more than two decades of Toyota Priuses, most people are familiar with hybrid cars. But there are a few different types of hybrid, some of which have only recently started to become commonplace. The most high-profile latter-day type is the plug-in hybrid (PHEV), but as automakers increasingly electrify their fleets, you may also be hearing more and more about mild hybrids (MHEVs) and 48-volt mild hybrid systems.

These terms can be confusing because, despite their familiar-sounding name, mild hybrids don't function or drive quite like other hybrids. Here, we've broken down how mild hybrids evolved, how they work, and how MHEVs differ from their full hybrid and plug-in cousins.

A mild hybrid or MHEV is a sort of half-step in between a traditional gas-powered vehicle and a full-on hybrid vehicle. The goal of a mild hybrid car is to consume less fuel, but it does so in a unique way that makes them less expensive than a full hybrid.

MHEVs use low-output electric motors typically fed by a small 48-volt battery. Their motors are actually integrated starter generators, which replace the typical alternator and starter motor. The integrated starter generator's electric brawn augments the engine when power is needed, like when you accelerate from a stop. Since the combustion engine doesn't have to work as hard, it consumes less fuel and generates less pollution. The integrated starter generator also acts as a seamless stop-start assist system since it can spin up the engine far faster than a regular starter.

The battery is then charged by recapturing kinetic energy typically lost during deceleration, or regenerative braking, which is also how full hybrids charge their batteries.

If there were ever two cars that could illustrate the differences between mild and full hybrids, it's the original Honda Insight and Toyota Prius. The Insight debuted in late 1999. The Insight was a mild hybrid, with an integrated starter generator mounted between the engine and transmission, which Honda labeled "Integrated Motor Assist." While Integrated Motor Assist couldn't drive the wheels, it did help boost the engine's output, allowing Honda to fit a smaller gas engine than it otherwise would have. Integrated Motor Assist could do most of what modern MHEVs do, and in 2006, Honda updated it to allow it to power a vehicle's systems while coasting, another trait of current MHEVs.

The Prius, which had premiered earlier in Japan, arrived in the U.S. for the 2001 model year. The Prius is a conventional (or "Series-Parallel") full hybrid, in which both the electric motor and the gas engine can directly drive the wheels and the car switches seamlessly between the two types of power.

Plug-ins (PHEVs) are an extension of full hybrids, with bigger batteries and beefier electric motors. A PHEV's batteries are so large that they can't rely on regenerative braking alone to charge, hence the plugs. The benefit of the plug-in hybrid is its ability to travel in all-electric mode for most short trips, and it reserves the gasoline engine for longer drives. In comparison, the battery of a mild hybrid (MHEV) is not large enough to propel the vehicle on electricity alone.

Honda used Integrated Motor Assist in its hybrids until 2015, but in 2007 it got a competitor from General Motors, the belted alternator starter (BAS) hybrid system. (Later updated versions were called eAssist.) Instead of mounting the electric motor between the engine and transmission, GM put its integrated starter generator on the outside of the engine, where the alternator normally goes, and hooked it up with an extra-strength serpentine belt. This system was less expensive to manufacture than Honda's, but GM never had much luck selling its MHEVs and subsequently dropped them in 2016.

Just as the Prius has stayed true to its formula, modern MHEVs still use these same ideas. Current Audi, Ram and Volvo mild hybrid cars and trucks use belt-driven integrated starter generators similar to the BAS system (and its later eAssist versions), while Mercedes-Benz's EQ Boost mild hybrids sandwich their integrated starter generators between the engine and the transmission like Integrated Motor Assist. Electrical technology, however, has since evolved.

Integrated starter generators need a lot of electrical power, as do lots of other modern vehicle components like driver assistance systems, electric superchargers and active suspensions. Today's cars have so many electronic gizmos that manufacturers are increasingly adding 48-volt electrical systems and batteries to them to compensate.

48-volt systems provide four times the power of traditional 12-volt systems, and higher voltages mean lower current draws and more efficient energy transfer. This allows automakers to use smaller wires in their spaghetti-like wiring harnesses, which saves weight and allows more regenerative energy to be recaptured. Higher-voltage systems could help even more, but 48 volts is considered "low voltage" by regulators, and systems above 60 volts — the "high-voltage" threshold — as in full hybrids, require more costly shielding.

The 48-volt setup is perfect for MHEVs, as it provides more than enough power but doesn't add too much cost or complexity. Honda's early systems used expensive high-voltage batteries like those in the Prius, while GM's BAS system at first used 36 volts, then 42 volts.

This depends on your needs and your driving style. MHEVs aren't as efficient as full hybrids or PHEVs, but they also cost less to buy, particularly compared to plug-in hybrids. That said, they do offer modest but meaningful gains in efficiency and performance over standard internal combustion engine vehicles.

MHEVs also provide extra electric oomph on hard acceleration, and they allow the combustion engine to shut off in several situations, so their efficiency and performance gains are skewed toward stop-and-go driving. As even the best mild hybrid systems can only yield about a 15% to 20% gain in efficiency, they also make the most difference in heavier, inefficient vehicles.

When the 5.7-liter V8 Ram 1500 pickup gained its eTorque mild hybrid system in 2019, it earned a 2 mpg increase in city and combined driving, and a 1 mpg increase on the highway, over the regular version's EPA ratings — a significant gain in a big pickup. The much smaller Volvo XC40 had a similar bump in city mpg when it became an MHEV, but only a 1 mpg gain in combined driving and no gain on the highway.

Tellingly, many brands that offer MHEVs are dropping their standard engine configurations to boost corporate fuel economy averages. Automakers clearly think mild hybrids are worth it.