The battery warning light usually means the charging system is not keeping up, not that the battery itself has failed. The most common causes are a failing alternator, a slipping or broken serpentine belt and corroded or loose charging connections. When it comes on while driving, the car is running on battery power alone and may stall when the battery runs down.
What the battery light means
The light comes on when system voltage falls outside the normal charging range, typically when the alternator output drops. On some vehicles it also comes on if voltage is too high, which means the alternator is overcharging. It normally lights briefly when you turn the key and goes out once the engine starts.
Common causes
- Failing alternator or voltage regulator; see bad alternator symptoms and P0562
- Slipping, worn or broken serpentine belt; see serpentine belt
- Loose or corroded battery cables, grounds or the alternator connector
- A blown charging fuse or fusible link
- A worn-out battery that the alternator cannot charge properly; see how long car batteries last
- Overcharging on vehicles that warn for high voltage; see P0563
What to do when the battery light comes on
- Turn off nonessential electrical loads such as the radio, heated seats and the rear defroster.
- If the temperature gauge rises or steering suddenly gets heavy, the serpentine belt may have broken; stop as soon as it is safe, because the water pump may also have stopped.
- Otherwise, drive directly to a safe place or a repair shop; you may have only a short time before the battery runs flat.
- Check the belt and battery connections once the engine is off and cool.
Can you drive with the battery light on?
Only for a short distance. A fully charged battery may run the engine for a limited time, less with headlights, wipers and the blower on, and modern engines stall once voltage falls too low. If the belt has broken, the engine can also overheat, so watch the temperature gauge and stop if it climbs. Plan to reach a safe place or a shop rather than finishing a long trip.
How to test the charging system
| Reading | Meaning |
|---|---|
| About 12.6 V engine off | Battery fully charged |
| About 13.5 to 14.7 V engine running | Alternator charging normally |
| Near 12 V or lower running | Not charging: alternator, belt or wiring |
| Above about 15 V running | Overcharging: regulator or sensing problem |
Battery light and other warnings together
Low voltage upsets other modules, so the battery light often appears with ABS, traction or transmission warnings and odd shifting. Fix the charging problem first, then recheck the other warnings; see ABS light and limp mode. If the battery is flat, see how to jump start a car.
More guides: car battery dies overnight · alternator or battery?.
Trouble codes: P0620 Generator Control Circuit · P0621 Generator Lamp Circuit · P0622 Generator Field Circuit · all OBD2 codes.
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