Warning lights
Battery warning light on: what it means and what to do
A battery warning light with the engine running means the charging system is not working: the car is using up the battery instead of recharging it. It can keep going for a limited time but will eventually stop, and the same fault sometimes affects cooling or steering.
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Reviewed by Daniel Morales, founder of Reprorace SL, on
Is it serious? Can I keep driving?
Carefully, and only as far as the garage. Drive straight to the garage by the shortest route, switch off what you do not need (climate control, rear screen heater, heated seats) and avoid stopping the engine. If the temperature also rises or the steering goes heavy, the belt may have snapped: stop as soon as it is safe.
A careful, general answer: only someone who inspects the car can say whether it is safe to drive. When in doubt, stop somewhere safe and call for assistance.
Most likely causes
- Alternator: a failed regulator or worn brushes stop it charging; this is the most common cause.
- Auxiliary drive belt: broken, loose or slipping: the alternator stops turning or turns too slowly.
- Terminals and cables: a loose or corroded terminal, or a poor ground cable, interrupts charging.
- Battery: a battery with a damaged cell can light the warning and run flat quickly.
- Smart charging control: on cars with a battery sensor or stop-start, a fault in the sensor or its settings.
Listed from more to less common in general; on your car they may be different. A fault-code reading and a check at the garage confirm which one it is.
To understand it better: How can you tell if the 12 V battery is failing? Why is stop-start not working? I have several trouble codes at once: which one should I fix first?
Car system: Electrical system and battery
What you can check yourself
With the engine off and cold, check that the belt is in place and free of cracks.
Make sure the battery terminals are tight and free of white or green deposits.
If you have a meter, with the engine running the system voltage is usually higher than the battery's resting voltage; if it is the same or lower, it is not charging.
Never disconnect the battery with the engine running: it can damage the electronics.
Safe checks only: with the engine off and cold whenever you look under the hood, without taking anything apart and without touching hot or moving parts.
Related fault codes
Generic codes that often come with this symptom. Seeing one does not confirm the cause: it is where the diagnosis starts.
How the ECO-LITE helps diagnose it
Plugged into the OBD socket, the ECO-LITE reads the fault codes of the control units that answer, with the freeze-frame data captured when the fault happened, and shows live data on your phone or computer. With your permission, your garage can see them remotely before you bring the car in.
What to look at here: system voltage measured by the control unit with the engine off and running, and how it changes when you switch on loads.
Frequently asked questions
How far can I drive with the battery light on?
It depends on the battery's condition and what the car draws: it may not be far. Do not count on more than a short trip to the garage.
Will a new battery fix it?
Only if the battery was the fault. If it is the alternator or the belt, the new one will go flat too. Check that the system charges first.
Why does it come on briefly when I start the car?
It is normal for it to light with the ignition on and go out once the engine starts. If it stays on or comes back while driving, there is a charging fault.
Keep reading
- Electrical system and battery: how it works and what fails
- Belt squeal when starting or turning: what it means
- Car battery keeps going flat: causes and what to do
- Car will not start and does nothing or just clicks
- Car symptoms: what they mean and what to do
- Check engine light on: what to do
- How to read and clear your car's fault codes
- OBD fault codes: what each one means