Why Do Headlights Flicker When Listening to Music?

Car with flickering lights

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The simple answer is power: your amp is drawing a lot of it, and the charging system in your car just can’t keep up. If it happens only when your car is idling, and only with really loud bass notes, then your situation is probably an edge case. You may be able to get away with installing a car audio capacitor, or stiffening cap. If it happens more often than that, or if you notice your headlights dimming even when the engine is “revved up” when you’re driving down the road, then something’s going to have to give.

Feeding Your Hungry Amplifier

Your powerful new amp is hungry, and what it craves is electrical current. The good news is that most cars produce more of that than they need, which is how your car can keep its battery charged even if you have accessories like headlights, windshield wipers, or your car stereo running. The bad news is that your alternator isn’t an infinite smorgasbord of juice. There comes a point where the rubber meets the road, and that point is often the installation of an amplifier — especially a powerful, dedicated subwoofer amp.

When you have a big subwoofer, and a powerful amplifier, the amount of current it draws is variable. If you listen to music that doesn’t have a lot of bass, then the amp isn’t going to have much of an appetite. That means you can crank up your AM talk radio stations or classical music all you want and probably never have a problem. If, on the other hand, you cue up your favorite Pandora radio dubstep station, that amp is going to get very hungry very fast.

Your Friend Flicker

So the root cause of your problem is that your amp is writing checks that your charging system can’t cash, and everything else suffers. In effect, your headlights dim and flicker because your amp is starving them. That leaves you with two basic solutions: fix your sound system, or alter your charging system.

If you like listening to loud music, and you probably do if you’re having this problem in the first place, the first solution is also the easiest. It’s also probably the least ideal from the point of view of someone who likes their music loud, since it only has one step, and that step is “don’t turn the volume up.” If you leave the volume down low, your amp won’t ever try to draw more power than the charging system is capable of putting out.

The other sound-system-related solution is to downgrade your amp. In the same vein as leaving the volume down low, installing a lower-powered amp will avoid the sticky problem of a charging system that isn’t quite ready for prime time. This is why it’s a good idea to check into the capacity of your charging system before you start upgrading your car audio, but you’re far past that point if you’re asking this question.

If you want to crank your music up with reckless abandon — without your headlights flickering — then you either need to upgrade your alternator or install a stiffening cap. The best solution is a bigger alternator, but you’ll have to talk to a qualified technician to verify that installing a high-performance alternator in your car is a viable option. Since other problems — like a failing alternator or bad wiring — can also cause dimming or flickering headlights, it’s probably a good idea to check with your mechanic anyway.