Battery Warning Light On in Your Car: What to Do Right Now?

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Car battery warning light on while you drive: pull over somewhere safe as soon as you can, because the alternator, the engine-driven part that generates electricity, is no longer recharging the battery. According to specialist sites, the engine can stall in roughly 15 to 30 minutes on battery power alone. The cause may be a belt, a terminal or the battery itself, and the checks take about 15 minutes.

Common symptoms and causes of a car battery warning light

When you turn the key with the engine off, the light comes on briefly. That test is normal, and the light should go out once the engine runs. If it stays on after that, it flags a fault in the charging circuit without saying which part is to blame.

• Alternator: regulator, brushes (the sliding contacts inside the alternator) or rotor.
• Broken or loose serpentine belt.
• Corroded or loose battery terminals.
• Worn-out battery.
• Charging fuse or relay.
• On a hybrid, the DC/DC converter (the unit that steps voltage down to the 12 V circuit).

A light that never comes on at startup would point, according to one specialist site, to a faulty bulb or circuit. If the car will not start at all, see our guide to a dead car battery.

Before you start

Have a multimeter ready for Solution 3, and keep your owner’s manual within reach. These checks apply to gasoline and diesel cars and mild hybrids: on a high-voltage hybrid or electric vehicle, do not take anything apart.

Warning: switch the ignition off, except when a step calls for the engine running. Do not touch the airbags, the seat belt pretensioners or the high-voltage system of a hybrid or electric vehicle, recognizable by its orange cables. A red warning light means you must stop the vehicle, and this procedure does not replace a diagnosis at a workshop.

Warning: the engine can stall and the power steering can fail. Pull over safely before any check.

Warning: a lead-acid battery can give off sparks and splash acid. Disconnect the negative terminal first if you need to touch the clamps. With the engine running, keep your hands away from the belt and moving parts, and run the engine outdoors.

Warning: never puncture, bend or heat a lithium battery; a swollen battery will no longer charge and must be taken to a collection point; work on a non-flammable surface.

Solution 1: red battery light while driving, the right reflexes

These steps come before any check.

1. Find a safe place to stop as soon as possible.
2. Switch off accessories such as the radio and the air conditioning if you still have to drive a little, as your manual advises.
3. Keep the engine running until you stop, because shutting it off and restarting on a weak battery is risky, according to a tester manufacturer.
4. Stop in a safe spot.
5. Call a repair shop.

Expected result: the car is stopped in a safe place and help has been called.

Carmakers do not all say the same thing. Mazda tells you to stop right away and not keep driving, because of the risk of stalling and an accident. Chevrolet, for the S-10, allows a short trip with accessories off, while noting that driving can drain the battery.

Good to know: according to specialist sites, power steering, ABS, ESP and the headlights can fade gradually on battery power alone. Follow the instruction in your vehicle’s manual. If it says nothing: stop as soon as possible.

Solution 2: the visual check, engine off

This check needs no tools. If you have to touch the terminals, disconnect the negative one first.

1. Turn the engine off, then open the hood.
2. Look for a serpentine belt that is broken, slack or damaged.
3. Check the alternator pulley.
4. Look for corrosion on the battery terminals.
5. Look for corrosion on the alternator and ground connections.
6. Check that the terminals are tight.

Expected result: the belt is intact and taut, the terminals are clean and tight. Then move on to Solution 3.

Tip: a white or greenish deposit around a terminal is corrosion. It is easy to spot from a distance, so there is no need to touch it.

Warning: a broken belt can also cut the power steering and cause overheating. Do not drive off. Call a tow truck.

Solution 3: measure the voltage with a multimeter

These values come from unofficial sources, so take them as guidelines. The threshold with the engine running varies between them from 13 V to 13.8 V.

1. Set the multimeter to measure voltage.
2. Place the probes on the battery terminals, engine off.
3. Note the voltage.
4. Start the engine, outdoors.
5. Place the probes on the terminals again, hands clear of the belt.
6. Note the voltage.
7. Compare your two readings with the guidelines below.

Engine off:

• About 12.6 V: fully charged battery.
• From 12.4 V to 12.8 V: normal range.
• Below 12.2 V: battery at the end of its life.

Engine running:

• From 13.8 V to 14.5 V: normal value.
• Below 13 V: insufficient charging.
• Same voltage as with the engine off, or lower: suspect the alternator.
• Above about 15 V: overcharging.

Expected result: a voltage that rises with the engine running shows the alternator is charging. If the battery is healthy and there is no charging, the alternator is the suspect.

A basic multimeter is all you need to take these readings.

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Measures voltage, current and continuity. Useful to check whether a fuse or a battery is in good condition.

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If the car will not start for the second reading, follow the checks for a car that will no longer start.

If the battery is too weak to start the car, here is what you need to get going again or recharge it.

Good to know: on recent cars, smart charge management can bring the voltage down toward 12.6 V with the engine running. According to a tester manufacturer, that is not a fault.

Solution 4: battery light that flickers on and off while driving or stays on

A light that flashes, or that stays on after a battery change, is traced in the same places. Start with the car stopped and the engine off. The last step clears the fault stored in the control unit: leave it to a garage.

1. Recheck the terminals for tightness and corrosion.
2. See whether the belt slips or lacks tension.
3. Keep in mind that worn alternator brushes can also be the cause.
4. Let the charge recover if you have just replaced the battery, as a tester manufacturer advises.
5. Also let the battery recover after a jump start: according to a tester manufacturer, the light can stay on until then.
6. Have a garage clear the stored fault with a diagnostic tool if the light stays on after the replacement: that is the lead given by a tester manufacturer.

Expected result: the light goes out with the engine running and does not come back. According to a tester manufacturer, it often goes out by itself after a replacement, sometimes after a drive cycle. If it persists, the cause has not been fixed.

If nothing works

Stop the checks and call a tow truck in these cases:

• the light stays on after the visual check and the voltage is below 13 V with the engine running;
• the belt is broken;
• you smell burning;
• a fault message appears.

A dashboard message reporting a charging system or 12 V system fault calls, according to one specialist site, for immobilizing the car quickly. Choose a tow over driving.

At the garage, explain when and how the light comes on, and give the voltages you measured with the engine off and running.

Tip: jot those two voltages down on your phone as you measure. You will not have to dig for them in front of the mechanic.

Take better care of your car: our toolbox

These accessories help you check the charging circuit and avoid being stranded again.

Frequently asked questions

Can you drive with the battery warning light on?

No, except long enough to reach a safe place. The Mazda manual says to stop right away because the engine can stall, and the Chevrolet S-10 manual mentions a short trip with accessories off. Follow your vehicle’s manual.

Why does the battery light come on while driving?

It flags a fault in the charging circuit without naming the faulty part. The alternator, the serpentine belt, the terminals, a fuse or a worn-out battery can be responsible. If it flickers or comes and goes, look at worn alternator brushes, a corroded or loose terminal, or a slipping belt.

The battery light is on but the car starts. Is it serious?

Yes, the fault still needs attention. The light points to the charging circuit, not to the car’s ability to start, and driving can drain the battery. Have the car checked right away.

Does the battery light mean the battery is dead?

Not necessarily. The light indicates a charging circuit fault, and the battery is only one part of it. According to a garage site, a battery lasts about 4 years on average, a useful benchmark if yours is old.

How can you tell if it is the alternator or the battery?

Measure the voltage with the engine off, then running: the battery and the alternator have to be tested together. If the battery is healthy, the alternator becomes the suspect. According to specialist sites, a voltage below 13 V with the engine running, or the same as with the engine off, or dropping, points to it.