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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

Capacitors

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.

  • Elizabeth from Sugar Land

    Posted on 6/30/2023

    After adding a aftermarket amp, sometimes the only speaker you here is the center dash and the added sub. I gave a 2020 Ford Lariat F150 super crew. Can you help.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 7/3/2023

    Elizabeth, It sounds like an interface device was not used in your vehicle's aftermarket amp installation and the factory system is reacting strangely to the additions. If you bought your amp from Crutchfield, you could call Tech Support for free help troubleshooting your system.
  • Gerald from San Antonio

    Posted on 6/16/2023

    Great article

  • David

    Posted on 6/15/2023

    Ok then you Say less than 1000 doesn't really need one I have the jl hd750/1 on two 8w7 and dim my headlights like crazy and was told the the big3 won't help they said turn bass down. Lol. What's the since in having subs.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 6/15/2023

    David, There are more than four ways to fix light-dimming - doing a Big Three is only one. Maybe you should try something else, like tightening connections or checking the battery.
  • Doug from Wisconsin

    Posted on 3/24/2023

    I have 2x RF T112D4s at 1 ohm with a T1000-1bdcp, and a P400x4 feeding 2x 50w 5.25's and 2x 60w 6x9s. Going in a classic car with a new 130a alternator. Wondering if I should exchange that now for 200a alt, or hook it up to see how it goes. I'll play it loud, but doubt I'll max it out often - if ever. Any help is appreciated!

    Commenter image

    Buck Pomerantz from Crutchfield

    on 3/27/2023

    Doug, To support that amount of power, your system may need an HO alternator and a second battery - but I'd wait and see if perhaps the system will work fine without any upgrades.
  • Kevin from Chicago

    Posted on 3/23/2023

    I found this article to be very informative with all options covered, being a former installer from the 80s.

  • Uzair Imran

    Posted on 1/3/2023

    I have a 2007 Nissan Maxima with stock 110 amp alternator and lead acid battery (replaced 1 month ago). I have a kenwood KAC-9106D amp that's 1000 watts x 2 and a kicker L7R 12" loaded enclosure that's 1200 watts x 2. Recently my battery and brake light flash when the bass hits and especially when I'm at a stop light. I'm guessing my stock alternator is dying (15 years old/140k miles). I've had this setup since May 2022 so I've been fine till the last couple of days. Can I upgrade to a 130 amp alternator without doing the big 3? Don't want to spend the money if I don't have too lol. I don't have any issues with light dimming etc. Thank you in advance!

    Commenter image

    Buck Pomerantz from Crutchfield

    on 1/5/2023

    Uzair, I don't know what battery and brake lights have to do with anything - unless there's a fault in your vehicle's electrical system. You can definitely upgrade the alternator without doing a Big Three wiring upgrade.
  • Connor Crist from San Diego

    Posted on 12/16/2022

    I have a 1500 watt amp for 1200 watt dual subs. I noticed that headlights start to dim and I hear popping from the subs when they dim so I know I need more power. I purchased a 300A alternator and the big three, I just need to get it installed. My question is, do I also need a new battery? I have seen some say that you do and others say you don't. I have a stock alternator in a '12 honda accord so the battery probably isn't the greatest. I am just wondering if I need to do battery upgrade because I dont want my car to give up on me nor my car audio keep popping and healights dim. Thanks!

    Commenter image

    Buck Pomerantz from Crutchfield

    on 12/16/2022

    Connor, You're apparently sending your subs 25% more power than they're rated for - so it's no wonder you're hearing popping noises - the voice coils are probably hitting the back plate due to being over-driven. Making sure your battery's up to snuff is one of the first things to check for when you have light-dimming - and it usually costs nothing to have someone check it for you. If your battery/charging system is okay, then you might consider adding an HO alternator and doing a Big Three.
  • Jose M Arce Jr from Oxnard

    Posted on 11/25/2022

    I got a 97 Lincoln Towncar with a JL audio RD900/5 ,the lights start dimming when i start to turn the volume up, 12" w3 HO enclosure and jl 5x7 and 6x9's,I was wondering if just buying that big XS 6500 battery would be enough or should I also do that big 3 + a high output alternator, i mean it's only 900 watts & 1000 watts,not anything crazy like 3-4000 watts?? i got another Lincoln Towncar 01 that im about to have installed a JLXD1000/5 v2 with a 12" HO w6 enclosure and jl 5x7 components and coaxials , my question would also be concerning this vehicle as well, any info / suggestions sent my way would be greatly appreciated, thanks!

    Commenter image

    Buck Pomerantz from Crutchfield

    on 11/28/2022

    Jose, Your 26-year old car may just need a new battery - have it checked out before deciding to do any other upgrade. Your 22-year old car may need the same thing.
  • Ryan from Indianapolis

    Posted on 11/13/2022

    Really from the standpoint of an auto tech I figure the big 3 would be before a second battery because there is no point on most cars because the thin ground cables some are as small as 12 guage and the alternators charge cable maybe 8 guage if your lucky choke even a stock alternator trying to pass 115 amps at 14.4 volts is over 1600 watts those little cables will melt trying to charge 2 batteries

    Commenter image

    Buck Pomerantz from Crutchfield

    on 11/13/2022

    Ryan, You make an interesting point, but the fused link in a car's positive power wire from the alternator will blow before a thin ground wire will burn.
  • Jason hall from Fort Myers

    Posted on 11/5/2022

    I wired up my car or my Ford Explorer with 4 gauge ofc amp kit now I want to do the big three and I don't know if I need to use the same type 4 gauge wire or should I go for a zero gauge wire? I'd appreciate any advice thanks!

    Commenter image

    Buck Pomerantz from Crutchfield

    on 11/7/2022

    Jason, The Big Three upgrade improves the ground and positive power connections in your vehicle and almost any gauge wire will work. Matching those wires to amp wire sizes isn't necessary - but is a good measure of what kind of power and ground wires will work best.

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