Belt-Starter Generators and 48V Mild Hybrids

A 48-volt mild hybrid system replaces the traditional alternator with a motor-generator — a single unit that can both generate electricity and produce torque to assist the engine. This unit is called a Belt-Starter Generator (BSG) when it sits on the accessory belt drive, or an Integrated Starter Generator (ISG) when it is sandwiched between the engine and transmission. It connects to a small 48V lithium-ion battery and a DC-DC converter that steps 48V down to 12V for the rest of the vehicle.

The BSG performs multiple jobs that used to require separate components. It replaces the starter motor — it cranks the engine for start-stop restarts so smoothly and quickly that you barely feel it. It replaces the alternator — it charges both the 48V and 12V batteries during deceleration (regenerative braking). It assists the engine during acceleration — adding 10 to 15 horsepower of electric boost from the belt to fill in turbo lag or smooth out shifts. And it enables engine-off coasting — the engine can shut off entirely during highway deceleration to save fuel.

RAM trucks use the eTorque system on both the 3.6 Pentastar V6 and the 5.7 Hemi V8 — this is a BSG on the belt. Mercedes-Benz uses EQ Boost with an ISG on most of their current lineup. Audi uses MHEV (Mild Hybrid Electric Vehicle) on Q7, Q8, A6, A7, A8 and others. Hyundai and Kia use Smartstream 48V MHEV on several models. BMW uses 48V mild hybrid on many models starting from 2020.

The 48V system is below the 60V threshold for high-voltage safety requirements, so you do not need HV gloves or special precautions. However, the 48V battery is a lithium-ion pack and requires proper handling — do not short circuit it, do not puncture it, and follow manufacturer procedures for replacement. The belt on a BSG system is more heavily loaded than a traditional accessory belt and may need earlier replacement. When the BSG fails, the vehicle loses start-stop functionality, regenerative braking, and the electric torque assist — it may feel sluggish and fuel economy will drop. DTCs will be specific to the BSG, DC-DC converter, or 48V battery.