Battery light on: causes and fixes

What usually causes the battery light to come on

The most common cause is the alternator. While the engine runs, the alternator makes power and charges the battery — and if it stops doing that, the battery light comes on. The battery itself can also be the cause: a battery that is worn out or has a loose connection can bring the light on too. On some cars a slipping or broken drive belt stops the alternator turning, which has the same effect.

The light can come on and stay on, or it can flicker. A light that stays on means the charging system is not working. A light that flickers can mean a loose connection or a belt that is slipping.

What to check by looking and listening

Look at whether the lights are dimmer than usual, and whether the heating fan is slower than usual — both point at the charging system. Listen for a whining noise that rises with engine speed, which can point at the alternator. Look at the battery terminals if you can see them: a loose or corroded connection can bring the light on.

If the battery light is on, the car is running on the battery alone, and the battery will run flat. Do not drive far with the light on — get the car looked at before the battery goes flat and the car stops.

Which part is usually involved

A battery light most often points at the alternator, and sometimes at the battery itself. Each one has its own page below.

• Alternators for your car
• Car batteries for your make and model
• Electrical parts — batteries, alternators and starters

Frequently asked questions