Alternator
The power plant on a belt
Once the engine is running, every watt the car uses — headlights, ECU, infotainment, recharging the battery — comes from the alternator. It’s a compact AC generator spun by the serpentine belt, with built-in rectification and a regulator that holds the system near 14 volts.
What it does
Generates electrical power while the engine runs and keeps the battery charged at ~14.4 V.
Hung on the side of the engine, spun by the serpentine belt.
How it works
- The belt spins a rotor that is an electromagnet, fed a small controllable current through slip rings.
- The rotor’s spinning magnetic field sweeps past the stator — three sets of stationary windings — inducing three-phase AC.
- Six diodes in the rectifier convert that AC into DC.
- The voltage regulator senses system voltage and trims the rotor current to hold ~14.4 V whether you’re idling with everything on or cruising with nothing on.
- A fan (often built into the rotor) pulls air through to cool the windings and diodes.
Key numbers
| Output | 70 – 180 A at ~14 V |
| Drive | Serpentine belt, ~2–3× crank speed |
| Rectifier | 6-diode three-phase bridge |
| Regulated voltage | 13.8 – 14.6 V |
Warning signs of trouble
- Battery warning light on while driving — charging has stopped.
- Dimming lights at idle that brighten with revs.
- Whining that rises with rpm — failing bearing or diode.
- Repeatedly “bad” batteries — often an undercharging alternator.
Maintenance & care
- Inspect belt condition and tension; a slipping belt undercharges.
- Keep connections clean — resistance at the main lug wastes output.
- Don’t jump-start carelessly; voltage spikes kill diodes.
Frequently asked questions
What does the alternator do?
Generates electrical power while the engine runs and keeps the battery charged at ~14.4 V.
How does a alternator work?
Once the engine is running, every watt the car uses — headlights, ECU, infotainment, recharging the battery — comes from the alternator. It’s a compact AC generator spun by the serpentine belt, with built-in rectification and a regulator that holds the system near 14 volts.
What are the signs of a failing alternator?
Battery warning light on while driving — charging has stopped. Dimming lights at idle that brighten with revs. Whining that rises with rpm — failing bearing or diode. Repeatedly “bad” batteries — often an undercharging alternator.
How do you maintain a alternator?
Inspect belt condition and tension; a slipping belt undercharges. Keep connections clean — resistance at the main lug wastes output. Don’t jump-start carelessly; voltage spikes kill diodes.