Battery warning light

The battery warning light is a red rectangle with a plus and a minus sign. It is normal for it to come on when you turn the ignition on and go out a moment after the engine starts. If it comes on while you are driving, the name is a little misleading: the battery itself is usually fine. The problem is that nothing is charging it.

While the engine runs, the alternator, driven by a belt from the engine, makes the electricity the car uses and tops up the battery. When the alternator or its belt stops working, the car runs on battery power alone. AAA warns that the car may keep running until the battery drains, then stall and not restart.

What to do right now

1. Switch off everything you do not need: radio, heated seats, rear screen heater, air conditioning and seat or mirror heaters. Keep headlights on if you need them to be seen.
2. Drive straight to the nearest garage if it is safe and close. Do not stop the engine on the way if you can avoid it, because it may not restart.
3. Watch for other warnings. If the steering goes heavy or the temperature light comes on, the drive belt may have snapped. Stop safely and call for help.
4. If you are far from help, stop somewhere safe and call your breakdown service rather than waiting for the car to die in traffic.

Likely causes, most common first

1. Failing alternator

   A common cause. Other signs, according to the RAC, include dim or flickering lights, electric windows or the stereo misbehaving, whining or grinding from the engine bay, and repeated flat batteries.

2. Slipping, worn or snapped drive belt

   The belt turns the alternator. A worn belt can slip and squeal; a broken one stops the alternator completely. A burning-rubber smell can point here.

3. Loose or corroded battery terminals

   A bad connection at the battery can stop charge getting through. If it is safe and you are wearing gloves, look for corrosion or a loose clamp.

4. Wiring fault

   A damaged cable or connector between the alternator and the battery can stop the charge.

5. Worn-out battery

   Less often, a battery at the end of its life can trigger the warning. If the car struggles to start as well, have the battery tested. See car battery replacement cost.

   Read the fix guide →

The symptoms of a bad alternator and a bad battery overlap, so ask a garage to test the whole charging system before replacing anything.

What it costs to fix

DIY or garage?

How to prevent it

• Ask the garage to check the drive belt at each service and replace it at the interval in your owner's manual.
• Keep battery terminals clean and tight.
• Have the battery tested every year once it is more than three years old.
• Get dim lights, a whining noise or slow starting checked before the alternator fails completely.