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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.

  • Brayden Burch from Warner Robins

    Posted on 7/17/2022

    I have a 400 watt 4 channel amp And a 500watt 2 channel amp that is only on when I flip a switch and I was thinking about Adding a 3000 watt amp for my subs. Do you think you will need a aftermarket alternator or a second battery. I drive a 1999 f-150 with a v8

    Commenter image

    Buck Pomerantz from Crutchfield

    on 7/17/2022

    Brayden, Aftermarket amplification with over 1000 watts of output will usually require an electrical system upgrade, like a second battery or HO alternator.
  • Jau'Quan Hughes from Inman

    Posted on 6/21/2022

    Hello Buck, I typically always go for somewhat powerful systems in my vehicles. My newest vehicle is a 2019 Kia Stinger GT. The factory Alternator in it is 180 amps , because it comes with a 15 speaker harmon Kardon system. I plan on adding my Rockford T2500bdcp and my T2S113. My first question is would you recommend still upgrading to a even higher output alternator (I usually do). Secondly the factory starting battery is trunk mounted so doing the big 3 is a bit more complex in this vehicle but I would like to do so because I know the sub and amp could drain the vehicles electrical significantly even though the factory alt setup is much higher than usual. Any recommendations or information on successful doing a big 3 on a stinger or other factory trunk battery vehicles ?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 6/23/2022

    Jau, Successfully installing an amplifier that powerful will always involve upgrading the vehicle's electrical system with at least a second battery and an HO alternator. A Big Three upgrade will improve current flow, but won't bolster the electrical system against excessive current draw.
  • Arthur Allen from Columbus

    Posted on 6/17/2022

    Hello I'm wondering do I need nd bigger alternator if I'm running two batteries and a capacitor...I'm trying to power an taramps 3k amp hasn't arrived yet...also is the capacitor and amp together not recommended?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 6/17/2022

    Arthur, Your vehicle will probably need an upgraded alternator in order to support an amplifier that powerful. A capacitor is always used in conjunction with a subwoofer amplifier.
  • Jeremy Kashif from Corryton

    Posted on 6/16/2022

    I have a 2017 accord coupe with the 7 inch touchscreen and "premium" sound system that isnt so premium. Ive disconnected the factory 8 inch sub and left it in place and added a kicker dvc 12" 43cwr122 in a sound ordinance bass bunker with a Jbl gtx500 wired with a Crutchfield ck8 amp kit. Since the stock head unit cant be replaced, i tapped the left and right rear speakers in the trunk with an audio control lc2i. It sounds pretty good but im getting major dimming at my headlights even at low volumes and at louder volumes the amp is cutting out completely. Ive checked my connections and they are solid so im wondering if I should look at adding a capacitor?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 6/16/2022

    Jeremy, Light-dimming at low volume and amp shut-down at high volume points to a serious connection issue. Double-check the power and ground wiring throughout your vehicle. Another cause could be a weak battery - that needs replacing.
  • Ricardo Bustos from Albuquerque

    Posted on 6/12/2022

    I have a 2016 Nissan Altima Sr And I'm only running 10 inch NVX VCW 102v2 to a NVX NDA102 1500 WATT 750 RMS Class-D 1-Ohm Stable Monoblock Amplifier. Will I need to upgrade my alternator? I just had to get a brand new battery for the car beacause the old one that came with the vehicle was stock I believe.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 6/13/2022

    Ricardo, A properly installed aftermarket audio system with that much power shouldn't normally bother the factory electrical system. But if your lights do dim on bass hits, then you will need an upgrade of some sort.

    Posted on 5/18/2022

    Hello Buck, I have 74 Pontiac Grandville with a 455ci. I have a 320HO alt. SQ3500 full range for 6 speakers. JL-audio 1000/1V2 amp for two Qclass 12's. I want to upgrade my starting battery. What would you recommend? With the size of my HO ALT could I get by with just an upgraded under hood battery? Or I need two?

  • Paul Wescott from WINCHESTR CTR

    Posted on 5/13/2022

    How do I get a high output alternator

    Commenter image

    Buck Pomerantz from Crutchfield

    on 5/13/2022

    Paul, You should go to an automotive repair and customization shop to get an HO alternator for your vehicle. A good shop will select the right parts to purchase and install them properly. Of course, you should get an estimate first, to judge expectations and have a basis for comparisons with other shop prices.
  • Norbert T Hubert from Sterling Heights ,Mi

    Posted on 4/29/2022

    A/C new install on a 1948 Buick Super. Has a 350 c.u, automatic trans. Would like an idea of cost for complete parts and install. Thank You Thank You Norb Hubert

    Commenter image

    Buck Pomerantz from Crutchfield

    on 4/30/2022

    Norbert, You'll have a lot more luck getting advice from an auto repair and customizing shop or online mechanic, than from the comments section of an article about car audio.
  • Joshua Moore from Brunswick

    Posted on 4/21/2022

    I recently had a 2001 civic I added a battery in the trunk to help with a Skar 1500 and a 15 inch L7. Pretty sure I destroyed the electrical system by not doing the Big 3...

    Commenter image

    Buck Pomerantz from Crutchfield

    on 4/22/2022

    Joshua, The lack of a Big Three Upgrade can in no way destroy a system. I'm pretty sure adding that huge amplifier out-drew the electrical system's capacity, even with the addition of a second battery.
  • Ian Pellegrino

    Posted on 4/18/2022

    My factory BOSE dims the headlights.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 4/19/2022

    Ian, You should take your car to the dealership or a repair shop to fix factory system problems.

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