Signs of a failing alternator: what to watch for

What it means

The alternator is driven by the engine, usually via a belt, and generates the electricity for everything running while you drive, plus the charge the battery needs for the next start. If it stops charging, the car can keep going for a while on the battery alone, but the battery steadily drains and the car will eventually stall or fail to restart.

Depending on the vehicle, battery condition and electrical load, that can be anything from a few minutes to considerably longer, and it happens faster with lights and climate control running.

Common causes

• A worn or failing alternator
• The alternator belt: lose the belt and the charging stops
• Wiring in the charging circuit
• On an electric car, the DC-DC converter that does the alternator's job

What to do now

1. If the battery light comes on while driving, cut electrical load such as the radio, AC and heated seats.
2. Get to a safe location or a workshop promptly. Do not assume it will resolve itself.
3. Watch the headlights at idle. If they are dim and brighten when you rev the engine, mention it.
4. Look at the belts for visible cracking or fraying.
5. If you need a jump to get going, have the charging system tested as well as the battery.

Alternator, or battery?

• Battery light on while driving: usually the charging system, meaning the alternator, the belt or the wiring
• Dim or flickering headlights, especially at idle: can point to a charging or connection issue
• Car stalls or will not restart after the light was on: the battery ran down with nothing recharging it
• A squealing noise on startup: a belt worth having inspected
• A slow crank when starting: battery, starter or connection, and a shop can test which

What people often get wrong

That the battery light means a bad battery. Despite its name, it usually points at the charging system, and a perfectly good battery is sometimes replaced when the real problem was a worn alternator belt.

When the car must go to a workshop

Alternator testing and replacement is a job for a professional. Because a weak battery and a failing alternator produce similar symptoms, a shop tests both rather than guessing.

Read next

• Battery light on while driving: what it means
• Car battery keeps dying: finding the real cause
• Car stalls while driving: what to do first

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.