What is the reason for the car battery light flashing on and off?

[CharlieFitz]

When the car battery indicator light flickers on and off while driving, it may indicate a malfunction. Possible causes: The intermittent illumination of the charging indicator light reflects unstable charging current from the alternator. The potential faulty components mainly include the alternator, voltage regulator, and power supply circuits. If the horn behaves abnormally when the engine speed is high, it suggests that the alternator's output voltage decreases at higher speeds. This could be due to weak alternator brush springs, where increased vibration at higher speeds leads to poor contact and voltage drop. Fault analysis: Excessive wear of the brushes may result in insufficient spring tension, causing poor contact and vibration between the brushes and slip rings, which can lead to slip ring burning. As the slip ring surface becomes rough, the poor contact condition worsens at higher alternator speeds compared to lower speeds. Consequently, the alternator may fail to generate electricity or produce lower voltage at high speeds.

[CarterLee]

Yesterday when I was waiting at a red light, the battery light on my dashboard flickered briefly, so I immediately pulled over to check. With older cars, the most common issue is the generator brushes wearing down—they're responsible for transferring the electricity generated by the alternator to the battery. If they're too short, poor contact can trigger a warning. At idle speed while waiting for a red light, the alternator may not produce enough electricity, causing the alarm. Another possibility is a loose or slipping belt, especially when the AC is on, increasing the load. I recommend that next time before a cold start, press the middle of the belt with your finger—if it sinks more than 1 cm, it needs tightening. If the light flickers frequently when the engine is warm, be cautious. My neighbor’s car had the light flickering for three days before it completely died, and it ended up needing a new brush holder after being towed in for repairs.

[VonNova]

My best friend's car had the same issue! Her battery light suddenly lit up for a second while listening to music. The mechanic said it was caused by a loose connection, such as oxidation on the generator terminal or a loose ground screw. Vibration caused it to momentarily disconnect and reconnect. Another common situation is battery terminal corrosion—it might look fine on the surface, but prying open the protective cover could reveal blue-green crystals all over. I taught her a trick to test it: Start the engine and use your phone's flashlight to record a video while observing if the headlights suddenly change brightness. If there's flickering between bright and dim, it's likely an unstable power supply, and you should check the wiring connections. Especially during the rainy season, pay extra attention to the battery terminals—they oxidize very quickly when exposed to water.

[ErinAnn]

Having worked in auto repair for twenty years, I've seen countless cases of battery warning lights flickering. The key is to differentiate the scenarios: brief flickering might just be a momentary voltage fluctuation, like a hiccup. But if the light stays on for more than two seconds, there's an 80% chance the alternator regulator is faulty. Here's a method you can try yourself—use a multimeter to clamp the battery terminals. A cold engine should read 12V normally; if it doesn't rise above 13.5V after starting, the alternator is in critical condition. Last week, a car came in with the light flashing. Upon disassembling the alternator, I found two rectifier diodes had shorted, causing the output voltage to drop directly from 14V to 11V. Actually, the light going off doesn't mean the problem is gone—it can recur like a chronic illness.