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Headlights dim when the music plays

How to give your car's electrical system a power-up

Headlights can dim when you need more power

Your headlights dim when the stereo pulls too much power out of your car's electrical system. This usually happens only in large, multiple-kilowatt sound systems. But sometimes even modestly powered systems can stress your car's electronics, especially during a sub amp's burst of energy when it produces the sound of the beat. Every part of the electrical system suffers because of the amp's sudden demand on the limited supply of power, even the amplifier itself. Your eyes are just more sensitive to the change than your ears, and you notice your headlights dimming along with the beat.

There are a few actions you can take to solve this problem. There are also some strong and differing opinions about the order in which you should do them, or even their individual usefulness. The following is my contribution to the argument.

T-Spec battery terminal

Sometimes all you need is a new battery terminal.

Check all power and ground connections, and the battery itself

The first thing you should do is make sure all the surfaces used in power and ground connections are scraped down to bare metal, clean, and all the connectors and the battery terminals fastened tightly together. If that doesn't do it, take your car and have its battery load-tested at an auto parts or battery store. They'll often do it for free and can recommend the proper replacement if they find your old battery has problems. Car batteries rarely live to be four years old, so don't think you're being cheated when they tell you your old battery isn't holding its charge. A weak battery can often have enough juice to start your car but not be able to handle the quick jolt of demand when your sub amp hits.

If you do decide you need to replace your battery, consider getting an XS Power battery from Crutchfield. XS Power batteries feature a sealed absorbed glass mat (AGM) design, instead of the standard liquid electrolyte design, so they actually store energy — meaning you don't have to add a capacitor to your system to get a quick burst when you need it. We carry a full assortment of the most popular sizes available.  We even have models which fit many European applications such as VW/Audi, BMW, Mercedes, and Jaguar.

XS Power battery

XS Power car batteries

Adding a second battery

The next thing to try is to add a second battery, often back near the amplifier. This, in effect, provides another source of power available to fill in the gaps when the system needs it. When the power is asked for, it comes from the batteries. Two batteries wired in parallel act like one battery with twice the capacity.

A second battery becomes almost necessary if you play your music a lot with the engine turned off. Some people use isolators between their batteries, so that the one used to start the car won't get drained by the amp's pull. And it's a good idea to only hook up two batteries of equal strength and age. The stress of two different strength batteries constantly cycling charge between themselves to equalize the voltage leads to a shortened life for both batteries.

XS Power BIG3XS big three wiring upgrade kit

XS Power BIG3XS big 3 wiring upgrade kit

The Big Three wiring upgrade

A good, cost-effective improvement is to perform the "Big Three" electrical upgrade. This will vastly improve your vehicle's electrical system, allowing more current to flow easier to all components. This upgrade replaces or augments three key cables in the electrical system with 1/0 or 4 gauge wires: the battery ground to chassis wire, the chassis to engine block wire, and the alternator plus to battery plus wire. This increases your electrical system's current flow capability, ensuring a more consistent voltage under varying demand conditions.

An unfortunate possible side effect of doing the Big Three is that occasionally it only makes your lights dim even worse. This happens because the amplifier is now able to suck juice better from the system through the bigger straw (the new cables).

High output alternator

The alternator is the ultimate source for all of your vehicle's power when it's running. The battery starts the engine, but the alternator takes over when the motor starts to turn. If your sound system demands more power than your stock alternator can supply, then you will benefit by upgrading to a higher output alternator. Replacing your stock alternator with an "HO alt" may seem like an expensive solution, but if you've invested in a sound system that draws this much power, you shouldn't mind solving this problem of supply and demand by increasing the supply of power.

For example: producing a 1500 watt output requires about 217 amperes of current from your car's system. It's actually about a third of that for music, which doesn't put out full power all the time — but it's still a lot of current.  Most stock alternators are in the 80-120 ampere range and can only supply about 40% to 50% of that for non-automotive uses like amplifiers.

A 250 or 300 ampere aftermarket high output alternator should provide enough power for all your car's systems and your high-powered stereo too. Keeping that in mind, calculating the required output size of a high output alternator is not easy and should only be done while consulting with the new alternator's vendor or installer.

T-Spec cap

T-Spec 3.0-Farad capacitor


Adding a capacitor to your system does not increase your system's power capacity. But if your lights dim only occasionally, and only on the loud drum beats, then a capacitor may just be the cure. A capacitor smoothes out the power demands by providing a short burst of energy when needed. I recommend 2 farads of capacitor for every 1,000 watts RMS of total amplifier power. That's more than the usual recommended dose, but it should ensure a quicker recovery time for the cap to be ready for the next thump.

Note: Working with a car's electrical system can be dangerous. Tools and jewelry can be welded by the inadvertent discharge of a battery or capacitor. Sparks could fly, igniting flammable gases. Batteries have been known to explode when overstressed. (I know it for a fact: I witnessed a friend's battery exploding and setting his Lincoln on fire.) If you're inexperienced or don't feel comfortable working with high-current devices, then hire a professional to do these upgrades and installations.

Only if you need to

And please, don't let anyone talk you into doing any of these procedures unless you are experiencing bothersome headlight dimming. Systems with less than 1,000 watts RMS of total output rarely, if ever, need the vehicle's electrical system upgraded.

  • Simeon J from Brooklyn

    Posted on 1/20/2022

    I have a 2018 Camry SE with stock alternator and a optima yellow top agm battery running a 2000 rms amp for bass and 600 and 400 rms 2 channel amps for mids and highs. I do not currently have any issues with dimming lights or anything of that nature but I am interested in upgrading my stock alternator as I want to increase my wiring size to zero gauge and do a big 3 upgrade in the summer. What size alternator would you recommend for my vehicle.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 1/21/2022

    Simeon, You'd do better asking that question at an auto parts store or a custom garage, than from us.
  • David from Pembroke Pines

    Posted on 1/6/2022

    Hi, I have a 2021 Toyota Corolla. I want to upgrade my system to a CXA1200.1 Kicker amp with a Dual Vented (DCWR122) 12's Comp R Enclosed 500 RMS each. My stock alternator is 100 amps. Is this a bad idea? The big 3 will be done during installation but is that enough?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 1/7/2022

    David, That amplifier is capable of drawing more current than your car's entire electrical system, and delivering more power than that subwoofer can handle safely. I recommend a smaller sub amp or a larger electrical upgrade than just a Big Three.
  • Michael J Handzel from Groveport, Ohio

    Posted on 12/23/2021

    When I step on my brakes, the power drain dims the headlights.. been that way since I resurrected this 1987 Jeep Comanche after sitting idle for six years. New engine and transmission and crutch field supplied the JVC stereo and tech help for wiring ... 30 k and 6 years later .. (no amp w/stereo) for Gods sake it's a small truck!) ( was kinda bummed that at 27-29 on volume it's distorted already) Headlights still dim as brake is applied..

    Commenter image

    Buck Pomerantz from Crutchfield

    on 12/23/2021

    Michael, Headlight-dimming when braking indicates a loose wire or weak battery/alternator system. Have an auto repair shop check your vehicle's electrical system. Make sure the stereo is off when doing so.
  • Fred Sittmann from McKinney

    Posted on 12/16/2021

    "Systems with less than 1,000 watts RMS of total output rarely, if ever, need the vehicle's electrical system upgraded." Unless it was manufactured in the previous millennium.

  • Eric B from Austin, TX

    Posted on 12/3/2021

    Hi Buck, I have a 2021 Dodge Challenger with a 160 Amp Alternator and 730 Amp Maintenance Free Battery. (stock) I was told by my dealer that upgrading my alternator would void my warranty. I was wanting to install 2 amps - 1200w (mono) running at 800w RMS @ 2ohm (Subs) and 1600w (4ch) running at 200 W MAX Power @ 4 Ohm x 4 Channels. Do you have suggestions for power? Would swapping the 2ohm subs (dual coil) with the 4ohm variant help? Thank you

    Commenter image

    Buck Pomerantz from Crutchfield

    on 12/3/2021

    Eric, Your vehicle will probably support the new sound system without needing an alternator upgrade. But if you do need an upgrade, you might want to wait until the new car warranty has expired before installing the upgrade, if you want to ever receive free repairs under warranty from your dealership.
  • Steve from Atlanta

    Posted on 11/18/2021

    1965 Buick. Currently getting (2) L7 Q series 12's, cxa1800.1 amp, cxa360.4 amp, and Sony 5500 double din head unit. Should I prepare for alternator upgrade? Thanks for any advice

    Commenter image

    Buck Pomerantz from Crutchfield

    on 11/19/2021

    Steve, I don't know about "preparing for" an upgrade, but you shouldn't be surprised if you need one after installing those amps. Systems over 1000 watts RMS in total power output often need help from an electrical upgrade.
  • Mike from Elk Grove

    Posted on 11/13/2021

    Hi everybody! I have a 2008 Mercedes E350 sport sedan and I'm considering getting some audio work done. I'm not sure how far I want to take it, but I have a good idea of where to start. I have a Skar Audio RP-800.1D and a single 12" 1,200 watt loaded vented subwoofer enclosure also from Skar. I want to throw the sub/amp in the trunk to compliment the existing factory setup for the moment. The amp has an RMS rating of 800w and the sub has an RMS of 600w. The stock alternator is rated for 180 amps which is nice, and I just put in a new beefy AGM battery. Would it be a good idea to get a big 3 upgrade done for this addition? Thanks all!

    Commenter image

    Buck Pomerantz from Crutchfield

    on 11/15/2021

    Mike, Generally speaking, you shouldn't have to implement any electrical system upgrades unless your vehicle's headlights dim on bass hits. Most systems with an output below 1000 watts RMS will not bother an electrical system.
  • Chuck from Oshkosh

    Posted on 11/12/2021

    Hey. I have a 2020 Impala and i currently have everything stock. On the verge of installing a 1500 RMS skar amp for a single 12 rated at 1250 watts RMS. Getting the big 3 upgrade with the install. Should i prepare to go further with a HO Alt or 2nd battery? Thanks

    Commenter image

    Buck Pomerantz from Crutchfield

    on 11/15/2021

    Chcuck, Generally speaking, you shouldn't have to implement any electrical system upgrades unless your vehicle's headlights dim on bass hits. After that, then you can implement the upgrades outlined in this article.
  • Mike from Chicago

    Posted on 10/28/2021

    Hey Buck - I have a 2021 5.7L Dodge Charger R/T with a 730 amp battery and 180 amp alternator, both stock. I have just about 2,500 watts of constant power in aftermarket stereo equipment. Big-3 is done and whole car is run with 0 gauge OFC wire. I'm experiencing severe voltage drop (dipping <10v every time the subs just start to hit). If I swap out my alternator with a HO 370amp one, would I also require a secondary or higher amperage battery? Thanks in advance!

    Commenter image

    Buck Pomerantz from Crutchfield

    on 10/29/2021

    Mike, Even without knowing what "constant power" means, I'd say that if you want your amplified system to be able to put out 2500 watts RMS, your car's electrical system should be able to supply it with at least 238A of current. That HO alternator may be enough of an upgrade to handle the power draw without a second battery - but with that much power involved, a second battery may be necessary.
  • james from owensboro

    Posted on 10/14/2021

    so, i have a 2005 poniac grand prix. i have 2 10 inch pioneers with a 600 watt kenwood amp. my lights dim and the vehicle will not function as well with them plugged in, suhc as all my guages dropping. if i disconnect them my car runs fine. should i do any upgrades, or just replace the stock battery and elternator with new ones?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 10/15/2021

    James, 600 watts shouldn't strain a car's electrical system. Go through the steps outlined in the article. I'll bet you've got a weak battery or bad ground connections.