What replaced what
On a conventional car the alternator is driven by the accessory belt and does one thing: it turns engine rotation into 12-volt electricity. A separate starter motor engages the ring gear to start the engine, and it is loud, slow and only used at the beginning of a journey.
A 48-volt mild hybrid deletes both. In their place is a belt starter generator — often written BSG or BiSG — which is a motor-generator bolted where the alternator was. It runs on a 48-volt network, and a DC-DC converter steps that down to feed the ordinary 12-volt system.
Because it is a motor as well as a generator, the belt now transmits torque in both directions. That single change is what makes the whole system work and it is also what makes the belt a different part.
What it buys
Silent, instant restarts. This is why start-stop went from irritating to unnoticeable. A BSG spins the engine up through the belt in a fraction of the time a pinion starter takes, without the clatter.
Torque fill. Roughly 10 to 20 horsepower of assistance, applied exactly where a small turbocharged engine is weakest — the moment between the throttle opening and the turbo making boost.
Coasting with the engine off. The engine can shut down while the car is moving, because it can be restarted instantly and the accessories are still fed from 48 volts.
Regeneration. Under braking the BSG becomes a generator with real load behind it and puts energy back into the pack, instead of the token amount an alternator recovers.
Why 48 volts specifically
Because 60 volts DC is the safety threshold. Below it, the system is not classified as high voltage: no orange cable, no service disconnect, no Class 0 gloves, no specialist certification to work near it.
That was the entire point. Manufacturers got a meaningful jump in electrical power — four times the wattage at the same current as 12 volts — without the vehicle becoming a high-voltage service problem.
How they fail
The belt. The obvious one and the most common. Under regeneration and restart the belt is being driven backwards by the BSG, so a belt that is stretched, glazed or contaminated slips in a way a conventional belt never would. The symptoms are a whine or a chirp on restart, a charging fault, or start-stop quietly ceasing to work.
Belt kits for these systems come as a belt plus the correct tensioner because the tensioner is doing more work than a conventional one, and replacing one without the other is a false economy.
The 48-volt pack. A small lithium pack, and like any lithium pack it ages. As it does, the system reduces what it will attempt — assistance goes first, then coasting, then start-stop, and eventually you get a warning. It is a wear item with a service life, not a lifetime component.
The DC-DC converter. This is the failure that confuses people. The converter feeds the 12-volt system, so when it fails you get a flat 12-volt battery, no-crank, and every symptom of a dead alternator — on a car that does not have one. Replacing the 12-volt battery fixes it for a day.
The BSG itself. Bearings and windings, like any electrical machine. Less common than the belt, and considerably more expensive.
Diagnosing it in the right order
- Look at the belt first. It is visible, it is cheap, and it causes most of the complaints.
- Check the 12-volt battery and its state of charge, because a weak 12-volt battery produces faults all over a system that assumed it would be healthy.
- Read the DC-DC converter's output before condemning anything upstream of it.
- Read the pack's state of health with a tool that can talk to the hybrid controller. A generic code reader will show you a fault code and not the number that explains it.
- Only then consider the BSG.
What this means for start-stop that has stopped working
On almost every one of these systems, start-stop is the first function the controller sacrifices when something is not right. That makes it a genuinely useful early warning: a car that used to stop at lights and now does not is telling you about the belt, the 12-volt battery or the pack, months before it tells you with a warning light.
Questions people ask
- Can a 48-volt mild hybrid drive on electricity alone?
- No. That is the distinction between mild and full hybrid. A 48-volt system assists the engine and restarts it; it has nowhere near the power or the energy to move the car by itself.
- Can I fit a normal serpentine belt?
- No. The belt on a 48-volt system is specified for reverse torque and usually comes with its own tensioner. A generic belt of the right length will slip under regeneration, which shows up as a whine and a charging fault before it shows up as a failure.