What Is a Car Alternator?
The alternator is an electrical generator driven by the engine via the serpentine belt. Its job is to convert mechanical energy from the engine into electrical energy (alternating current, converted to DC) that recharges the car battery and powers all of the car's electrical systems while the engine is running.
Without a working alternator, the car relies entirely on the battery — which will go flat, typically within 20–60 minutes of driving, depending on electrical load.
How Does an Alternator Work?
Inside the alternator, a rotor (a spinning electromagnet) rotates inside a stator (a set of fixed copper windings). As the rotor spins, it induces an alternating current in the stator windings. A rectifier converts this AC current to DC, and a voltage regulator keeps the output at a steady 13.5–14.5 volts — the correct range to charge the battery without damaging it.
The alternator is driven by the engine's crankshaft via the serpentine belt (or, on older vehicles, a dedicated V-belt). If that belt breaks, the alternator stops generating power immediately.
Signs of a Failing Alternator
| Symptom | Notes |
|---|---|
| Battery warning light on dashboard | Classic sign — get tested promptly |
| Dimming headlights or flickering dash lights | Voltage dropping as battery drains |
| Electrical accessories behaving oddly | Radio resetting, windows slow |
| Whining or grinding noise from engine bay | Worn alternator bearings |
| Battery goes flat despite being new | Battery is fine; alternator isn't charging |
| Burning smell | Overheating windings or slipping belt |
A common mistake NZ drivers make is replacing the battery when the alternator is actually the fault — only for the new battery to go flat within days. Always test both when experiencing starting or charging issues.
Consequences of Driving with a Failing Alternator
If you continue driving with a failing alternator:
- The battery will gradually discharge
- Electrical systems will start shutting down (power steering on some vehicles, ABS, airbags)
- The engine will eventually stall and not restart
- In extreme cases, the alternator can seize, snapping the serpentine belt and disabling the water pump — causing overheating
This is not a "limp home" situation. If the battery light comes on and stays on, pull over safely and arrange a tow or roadside assistance.
Testing the Alternator
A mechanic can test the alternator output with a multimeter or a dedicated charging system analyser in minutes. You can do a rough check yourself: with the engine running, a healthy system should read 13.5–14.5V across the battery terminals. Below 13V indicates insufficient charging.
NZ Repair Cost Estimates
| Repair | Typical NZ Cost |
|---|---|
| Alternator replacement (reconditioned unit) | $350–$600 fitted |
| Alternator replacement (new unit) | $500–$950 fitted |
| Serpentine/drive belt replacement (if also failed) | $120–$280 additional |
| Voltage regulator replacement (some vehicles) | $150–$350 |
Remanufactured alternators are widely available in NZ and perfectly reliable for most applications. New OEM units are typically only necessary for high-mileage utes or performance vehicles.
When to Book a Mechanic
Book a mechanic as soon as possible if:
- The battery warning light illuminates and does not go off
- Your headlights or interior lights are visibly dimmer than usual
- You hear new grinding or whining noises from the engine bay
- You've replaced the battery but it keeps going flat
Don't delay — a failed alternator can strand you and, in the worst case, lead to an expensive tow and engine damage from overheating.