In this article
A 48-volt mild hybrid adds a small electric motor-generator and a 48-volt battery to a conventional gasoline or diesel engine. The motor restarts the engine smoothly after automatic stop-start, recovers energy when slowing down, and adds a short boost of electric torque when accelerating. Unlike a full hybrid, it cannot drive the car on electricity alone for meaningful distances. Luxury brands use it widely — the current Mercedes-Benz S-Class uses it in every version — mainly because it makes cars smoother and more efficient.
At a glance
| Mild hybrid (48V) | Full hybrid | Plug-in hybrid | |
|---|---|---|---|
| Electric-only driving | No (or very limited) | Short distances at low speed | Many miles |
| Battery size | Small | Medium | Large |
| Plug in? | No | No | Yes |
| Main benefit | Smoothness, modest efficiency | Big city efficiency | Electric commuting |
How it works
- Starter-generator: a belt-driven unit or an integrated starter-generator between engine and transmission.
- 48-volt battery: stores energy recovered during braking and coasting.
- DC-DC converter: links the 48-volt system with the car’s 12-volt system.
Why 48 volts?
Twelve-volt systems cannot deliver enough power for these functions, while high-voltage (several hundred volts) systems require expensive safety measures. Forty-eight volts is powerful enough to be useful while remaining below the threshold that requires extensive high-voltage protection.
What passengers notice
The biggest difference is refinement. Older stop-start systems shuddered when the engine restarted at a traffic light; with a 48-volt starter-generator, the restart is nearly imperceptible. The electric boost also fills in the moment before turbochargers build pressure, making acceleration smoother.
Other uses of 48 volts
The 48-volt system also powers energy-hungry features, such as active anti-roll systems on the Bentley Bentayga, electric superchargers on some Audi models and E-Active Body Control on Mercedes. Read Magic Body Control explained and the Bentley Bentayga.
Compared with hybrids and EVs
Mild hybrids are a step toward electrification rather than an alternative to it. Read hybrid vs plug-in vs electric and S-Class engines explained.
Good to know
Frequently asked questions
What is a 48-volt mild hybrid?
A system with a small electric motor-generator and 48-volt battery that assists a combustion engine but cannot drive the car on electricity alone.
Does a mild hybrid need charging?
No. It recharges itself through braking and the engine.
How much fuel does a mild hybrid save?
Savings are modest and vary by driving; they are greatest in stop-and-go traffic.
