Why Car Audio Capacitors Don't Work

Updated on December 20, 2019
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I love science and I enjoy trying to find out how the world works.

Capacitors are not ideal for the sound system in your car.
Capacitors are not ideal for the sound system in your car. | Source

Many car audio fanatics will use a power capacitor as an alleged passive storage device to supply current to their amplifiers. Capacitors are advertised to act as a supplemental power supply between your car’s electrical and audio system. They typically come in cylindrical shapes that are three inches in diameter and about a foot long, though other shapes are also available. They have a positive and negative terminal. Some capacitors may have an additional remote turn-on terminal that turns an LCD display on or off.

I can tell you that these devices do not work as advertised. Capacitors are electrical buffering devices. They do not generate an actual current.

However, many people like to use these devices for their sound system. They also swear that their flickering headlights went away or were reduced after the addition of a capacitor. We will explore this phenomenon in detail.

Why Are Capacitors Useless in Car Audio Systems?

Capacitors don’t produce any current on their own. This shouldn’t be news to anyone. Their purpose is to stabilize voltage by acting as a virtual “voltage wall” (buffer) to their intended devices. But they have a very detrimental side effect when applied to current-limited automotive systems. They buffer current flow behind the “voltage wall” and redirect it away from their intended devices: the amplifiers!

Your car audio system runs mainly off of your alternator, not your battery. This is because your battery has a voltage potential of 12.7 V, whereas your alternator has a voltage potential of 14.5 V. Since the alternator has the higher potential, it will supply the demanded current up to its point of saturation. When this occurs, the battery takes over to supply the extra spikes of current demand. But in such a saturated scenario, the voltage supplied to your audio system would have dropped substantially. The addition of extra batteries does not actually solve the problem. There is only one solution—you’re gonna need a bigger alternator!

You might consider adding a big capacitor, perhaps something around three farads. This will not help your situation. It really doesn’t matter how big the capacitor is. In fact, a bigger capacitor can lead to bigger problems. Bigger capacitors have longer charging times. This means that they will draw more current from your alternator for longer periods of time. During this process, your amplifiers will be starving for power because their voltage has dropped. When their voltage drops, so does their current.

You may think a capacitor is supposed to prevent this, but this is not the case. A capacitor cannot prevent this in a limited current environment such as a DC car audio application. Your alternator is your limiting factor here because it's working overtime. The reason you installed a capacitor is because you were already approaching the limits of your electrical system. This means that your alternator is reaching its limit when you turn on your audio system, A/C, headlights, defogger, wipers, horns, navigation, TV/DVD, and any other device. Now that you have added a capacitor into the mix, your alternator will have an additional labor-intensive job to do. The alternator now has to redirect a portion of its limited current supply away from your audio system and charge your extra capacitor! Of course, this doesn’t happen with home audio amplifiers because their current source is unlimited for all intents and purposes (due to high voltage). This is the reason why some home audio amplifiers can have built-in banks of buffering capacitors that don’t cause a decline in voltage and current.

Due to the fact that automotive electrical systems have a very limited current capacity (usually around 70A–120A), the addition of a capacitor will do nothing more than add more stress to an overworked electrical system.

Why Do My Headlights Flicker Less With the Capacitor?

Many people will use a capacitor because their headlights flicker at night when their audio system is pounding. They figure that the capacitor will supply these high current demands directly to their amplifiers. Therefore, the amplifiers won’t stress the rest of the electrical system, including the headlights. And guess what? When they add the capacitor, the headlight flickering is reduced substantially. So there you go—this is “proof” that the capacitor performed as advertised!

Not so fast! Let’s analyze exactly what is happening when you add a capacitor between your amplifiers and the rest of your car’s electrical system.

Since the capacitor’s positive and negative terminals are directly connected to the terminals of your amplifiers, it actually acts as a buffer for the headlights—not for the amplifiers!

The accessories (headlights) and the current output by the alternator are behind the “voltage wall” of the capacitor. The current generated by the alternator is directly available and is pulled by the car’s accessories due to the path of least resistance and the buffering action of the capacitor. This happens because the capacitor’s voltage drops when it's drained by the amplifiers. Since the voltage potential is higher at the alternator end, which is behind the capacitor, the accessories have a more stable voltage and current supply than the amplifiers do as the capacitor is slowly charging. The amplifiers are directly connected to the capacitor, so they will also see a corresponding drop in voltage on their terminals, whereas the headlights won’t (because they are connected to the alternator and battery). Therefore, the capacitor will send less of a current than what the amplifiers are demanding. This is because there is a limited amount of current available in this automotive system. The voltage on the amplifiers will fluctuate with every discharge and charge of the capacitor. But the accessories behind the capacitor won’t see such a drastic fluctuation in voltage or current. This is what is reducing the flickering of headlights. The capacitor is actually reducing the spikes of current demanded by the subwoofer amplifiers each time the subs hit hard. This means that your audio fidelity is somewhat compromised.

Yes, the amplifiers will instantly use a minimal amount of current available from the discharging capacitor, but this is substantially outweighed by the negative side effects in the whole circuit.

Why You Should Not Use a Capacitor in Your Car

  1. The capacitor’s current is very limited in duration (time), magnitude (amperes), and in quantity (amp-hours). It is not enough to maintain durations of extreme audio fidelity demanded by the amplifiers.
  2. There is an instant drop in voltage and current to the amplifiers when the capacitor is re-charging itself.
  3. The amplifiers are directly connected to an unstable supply of incessantly fluctuating voltage and current—the capacitor!
  4. The capacitor is an added load on the alternator, thus stealing current away from your amplifiers and recharging itself.
  5. The capacitor severely limits current to the amplifiers while it is charging.
  6. It takes milliseconds to discharge the capacitor, but it takes much longer to charge it, especially as it ages in the extreme automotive environment.
  7. The capacitor acts like buffer for your accessories, not your amplifiers. It restricts the current being sourced to your amplifiers because there is a limited supply of it. You don’t have an AC home outlet in your car with high voltage and a virtually unlimited current supply.

But My Headlights Are Flickering, What Can I Do?

Whatever you do, don’t add a capacitor! It’s a waste of money. You are better off putting your money towards the big 3 upgrade to your alternator’s wiring using 0-guage stranded wire. This can frequently solve most of the headlight flickering problems while still maintaining the fidelity of your audio system. If the big 3 upgrade doesn’t reduce your headlight flickering issue, then you have saturated the output stage of your alternator. An additional battery may slightly help your situation, but it will place added charging stress to your alternator and most likely reduce its life. The last resort is to install a high-output alternator of 160+ amps. This will remedy all of your electrical problems, but it will require the big 3 wiring upgrade.

The current produced by the car’s alternator is difficult to get past the capacitor and onto the amplifiers. The pernicious capacitor steals current from the amplifiers and gives it to the accessories (such as lights, ignition, A/C, and defogger). It acts like a fluctuating “voltage wall” that prevents a portion of the alternator’s available current from ever reaching the amplifiers. A capacitor does not produce any additional current in the system. It just redistributes current in a way that is detrimental to the audio fidelity of your system.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.


Submit a Comment
  • profile image


    15 months ago

    You are correct, sir.

    Any stabilization of "current"(A=W / (√3 × PF × V) draw in the scenario/s mentioned will be achieved only by at least a 100amp alternator.

    Very good article, thank you.

  • profile image

    Jamie Collins 

    16 months ago

    Even though its old, this article just got under my skin. It reminds me that the internet is an outlet for everyones opinion good, or bad.

    First, you make many assumptions about peoples usages, and setups, and further generalize every piece of equipment, and every person as one and the same.

    Second it appears you lack some crucial information about Car audio capacitors. more later.

    Third I want to go ahead and rebut your list:

    1. The capacitor’s current is very limited in duration (time), magnitude (amperes) and thus in quantity (amp-hours).

    Answer: yup, so is your alternator, and battery. (no matter how many you have)

    2. There is an instant drop in voltage and current to the amplifiers when the capacitor is re-charging itself.

    Answer: not exactly, if the capacitor isnt completely discharged, voltage on the amplifier side should stay high.

    3. The amplifiers are directly connected to an unstable supply of incessantly fluctuating voltage and current—the capacitor!

    Answer: yup, and so they are also without a capacitor, in fact even more unstable. In fact even though not installed as a shunt, the cap will stabilize current fluctuations, such as surge current drops seen when you turn on an accessory such as the headlights.

    4. The capacitor is an added load on the alternator, thus stealing current away from your amplifiers and recharging itself.

    Answer: The first part is written out of context. the second part is incorrect. Any component that uses or stores electrons in an electrical system is a load. Correction, a storage device is only a load if its charging. The statement that the capacitor 'steals' electricity from the amplifiers is factually incorrect provided an appropriate capacitor is being used.

    5. The capacitor severely limits current to the amplifiers while it is charging.

    Answer: Factually incorrect provided an appropriate capacitor is being used.

    6. It takes milliseconds to discharge the capacitor, but much longer to charge it; especially as it ages in the extreme automotive environment.

    Answer: Mostly accurate, but you liken the cap as a bucket you dump water out of in a split second, and then wait minutes as the garden hose refills it. Most caps do discharge faster than the recharge, but I think the time frames in your mind are far greater than in reality. Its unlikely this will affect the normal operation of your amplifiers provided you system is properly setup.

    7. The capacitor acts like buffer for your accessories, not your amplifiers. It restricts the current being sourced to your amplifiers because there is a limited supply of it. You don’t have an AC home outlet in your car with high voltage and a virtually unlimited current supply.

    Answer: Does act as a buffer for electrons. Doesnt restrict current to amplifiers.

    Explanation: You arguments are basic, but understandable because I think you lack some fundamental understanding of how these type of capacitors work. Traditionally electrolytic caps in typical electronics were used as filter caps. When powered on surge current fills the plate in the cap and when current flows in the circuit it flows at an expected rate. This is because the designer of the circuit knows what the load is on the other side of the capacitor. They will then select a capacitor size that will ALWAYS be 'full'

    These audio capacitors are known as 'fast recovery' they discharge and charge very quickly. One very important thing I notice you are missing is your understanding seems to be that these devices charge THEN discharge THEN charge again. when in fact they are doing BOTH at the same time. (sorry I use CAPITOLS in lieu of italics)

    So in fact when the capacitor is completely discharged, it is at that point simply passing the available current to the amplifier as if it wasnt there at all. With One exception. ESR. ESR is electrical resistance; however these capacitor types have very low ESR. So if your capacitor is staying completely drained all the time when your music is playing, then yes, it is in fact worse than not having a cap. it also means your circuit is incorrectly designed.

    Furthermore, if your capacitor is completely drained, more than (just a guess) %50 of the time, you have incorrectly designed your circuit.

    Which is the whole problem, and probably why you think car audio capacitor dont work.

    They do work, and probably do exactly what the manufacturers say they do ALBEIT they probably grossly overstate the capabilities of their particular product. Such as a 500 watt speaker that is only 75 watts! Now you get it? Assuming the problem is what it says it is ie. 2 Farad (whatever) then the problem is with the circuit designer. You!

    One of the biggest problems I see in car audio systems is that the industry has led every joe blo to believe if you just buy this, and this, and this, and that, plug them all together, and BAM! you have an awesome competition system!

    When in FACT a car audio system is an electrical/electronic CIRCUIT, and when people start thinking about it this way it becomes much more logical than: ok, my headlights dim when the bass hits, grab a capacitor and plug it in.

    Conclusion: Assuming a car audio capacitor is the right type, size, rating, properly installed, and used within its design parameters. They will function as intended. Period.

    '99 Virginia Tech B.S Computer Electronics

    '97 Virginia Tech B.S Mechanics

    '91 Nashville State A.A.S Automotive Tech

    Four years bench tech consumer electronics repair (TV's, Stereos, CAR AUDIO AMPS, DVD players etc)

    Two years on site tech Equifax

    Two years Level 1 tech Raytheon (DOD project)

    Two years home based computer repair business

    30+ years working on passenger cars and light trucks (two years as a professional mechanic)

    Disclaimer: I dont work for any company or industry for car audio, electronics. I own a business in the construction industry, that has nothing to do with cars, audio, or car audio. I dont endorse any product for anyone.

  • profile image


    22 months ago

    This is a terrible article. First, your diagram is WRONG. Second, capacitors smooth out the dips that cause your lights to dim or your stereo to buzz if you charge your phone while it's jacked in.

    You sound like you didn't even spend 5 minutes researching this topic.

    I bet you delete this comment instead simply admitting you are wrong. Probably where the rest of the comments went, lol...sad

  • profile image


    2 years ago

    Just curious as to why these results tend to show and increase in wattage at 4, 2 & 1 ohm loads versus without a capacitor? I did notice a decline in power with the higher farad cap versus just the 1 farad in this testing.



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