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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.

  • 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.
  • Warren from FLINT

    Posted on 10/8/2021

    Hello I upgraded my 10 Lucerne with a 250AMP ALT and a Brand Interstate Battery. Then I added an XS Power D1200 in the trunk. All of these additions call for a Big 3 upgrade so I bought 0 Gauge cables but it seems to only hurt my system. starts at 15.3 and After About 10mins the voltage dropped to 12.8 and fluctuates throughout the day never gives me power when I need it. so after months of this issue I finally took the big 3 off and left the additional ground wires I added and now my system works so much better thru my volt meter I can actually see my Voltage recharge rapidly when the bass hits now. Do you have info as to what may have been the issue with my big 3 power from battery to alternator wire. it is brand new no cuts holes or kinks. brand new terminals connectors. this vehicle is actually very hard to ground properly I had the worst time finding good grounds the whole car is basically aluminum GM clearly seen this as and issue so every ground they made has a metal bracket between the ground and the body. I just used those same grounds for my car audio additions. my voltage still doesn't hold the way it should but it does recharge very quick the way it should with the extra battery and bigger Alt. any insight you have would be greatly appreciated thank you!

    Commenter image

    Buck Pomerantz from Crutchfield

    on 10/12/2021

    Warren, "An unfortunate possible side effect of doing the Big Three is that occasionally it only makes your [voltage drop] even worse. This happens because the amplifier is now able to suck juice better from the system through the [larger cables]." It seems that your vehicle's electrical grounding system did benefit from the upgrade.
  • Scott Brown from St.Regis

    Posted on 9/16/2021

    I have a 6000 watt system can NOT GET the full bass without car trying to stall at a stop! 1999 pontiact grand prix 3.8 I running off battery and alt. Same time

    Commenter image

    Buck Pomerantz from Crutchfield

    on 9/17/2021

    Scott, A system this large will need upgrades to the vehicle's electrical system, including a second battery and an HO alternator.
  • Ryan Parton from Puyallup, WA

    Posted on 8/27/2021

    I have a 2016 Dodge Challenger Scat Pack. I believe it has a computer controlled charging system. If I run a 1/0 gage wire from the alternator directly back to the battery would it ruin the PCM or charging system in the us vehicle? And can I run the ground cable from the alternator mounting bolt to the body of the vehicle to count as the block to body ground

    Commenter image

    Buck Pomerantz from Crutchfield

    on 8/27/2021

    Ryan, It sounds like you want to perform a Big Three upgrade to your electrical system, without explaining why. As long as you leave the factory wiring intact and just add wiring, the computers will survive just fine.
  • Bob from SOUTH WINDSOR

    Posted on 8/10/2021

    I have a 2008 Honda CRV all stock electrical system. I'm putting in a Kicker mono sub amp that supports 150 Watt @ 4ohms or 300 Watt at 2ohms. I have old single subs 10" that are rated for both. If I use the 2ohm sub @300Watt does that actually pull twice as many amps too? Or is the watt output result pulling the same amount of amps between 2ohm and 4 ohm. I'm asking because this will affect how powerful a 4 channel amp I can buy for the door speakesr. I intend to stay well below the 1000 watt limit so I don't have to upgrade the electrical system. The battery in my car is very small almost like a motorcycle battery size.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 8/12/2021

    Bob, The impedance load of a sub is only relevant to an amp's current draw in respect to how it affects its output level. No matter what the load is, if you expect a Class D amplifier to put out 300 watts RMS, it will have to draw about 400 watts RMS - 29 amperes at 14 volts - from the vehicle's electrical system.
  • Miguel from Foley

    Posted on 7/28/2021

    2013 hyundai sonata..700 watt skar amp..10" display 1? (12) Gothic sub..will a 220 amp help my voltage issue.?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 7/29/2021

    Miguel, To ensure you get the right HO alternator, or second battery, for your system, we recommend taking your vehicle to an auto repair shop that specializes in electrical upgrades.
  • Jay S from Los Angeles

    Posted on 7/23/2021

    Hi I drive a 2011 Hyundai Sonata and my last DB drive 3k rms 1 ohm amp failed after a long session of high bass volume. Lasted two or three years. I have a 3000 rms DB Drive 12Wdx G5 Subwoofer and am struggling to find out which amplifier and HO alternator/battery(ies) combo to use. I think the biggest HO alt I can use is 220 amps. How strong of an amp can reliably be pushed by that?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 7/23/2021

    Jay, One of the rules of thumb for car audio power is that one-half of the alternator's total output is available for powering aftermarket equipment. An aftermarket HO alternator contributes a bit more power than that. In your case, I'd say a Class D amplifier that can output up to 1500 watts RMS at 2 ohms will work well.
  • Andrew from Spokane

    Posted on 7/17/2021

    I have a Honda Fit 2016. I've ordered the audio gear and everything will equal 700 watts rms (maybe slightly more). However, the battery in my car is smaller than the average battery and I cannot find a battery anywhere that is at least 700 cranking amps that will fit in my car. I ordered the XP750 to use as a 2nd battery. I'm thinking I'm going to have to get a battery isolator to prevent the batteries from killing each other sooner since they don't match. Is this the best move or should I do something different?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 7/19/2021

    Andrew, Most vehicle electrical systems can support an aftermarket stereo of up to 1000 watts RMS without needing an upgrade. If yours does, maybe you should consider taking your car to an auto mechanic for installing a second battery, especially if it doesn't "match" the original battery.
  • Joseph from San Antonio

    Posted on 7/9/2021

    Buck. I have a 94 mustang gt 5.0 And plan on installing my 1200watt peak ( 563 Watts RMS x 1 @ 4 ohms) A/B class monoblock. To power my 12 in dvc sub. My stock alternator puts out 130. From what ive read. This should be enough. But my alternator or battery may be bad. I plan on checking the two 1st to see each ones status. Then finishing checking all the grounds to make sure they are cleaned. And adding an extra power wire from the battery to the alternator. I've already added an extra ground to the alternator coming from the batter. (For the battery)

  • Franklin from Aruba

    Posted on 7/9/2021

    I have sprak 2012 i one now if i can upgrade .y altinator 250 amp if that can efect my car computer ?im from ARUBA

    Commenter image

    Buck Pomerantz from Crutchfield

    on 7/12/2021

    Franklin, A professional auto mechanic should be able to upgrade your car's electrical system without damaging its onboard computer system.

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