Why Car Audio Capacitors Don't Work


Many car audio fanatics will use a power capacitor as an alleged secondary, passive storage device to supply current to their amplifiers. The capacitor is 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 and possibly a third remote turn-on terminal that turns an LCD display on or off.

Q: Do these devices work as advertized?

A: No! Capacitors are electrical buffering devices. They are not current-generating devices.

But wait!

Many will swear that their headlight-flickering issues 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?

Because they don’t produce any current in and of themselves. 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!

Remember: Your car audio system is running mainly off your alternator, not your battery because its voltage potential is 12.7 V, whereas your alternator supplies 14.5 V. Since the alternator has the highest potential, it will supply the demanded current up to its point of saturation, whereby 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 has dropped substantially. The addition of extra batteries is only a limited band-aid fix to the problem. There is only one solution: You’re gonna need a bigger alternator!

What about adding a big capacitor...3+ Farads?

It won’t help your situation. It really doesn’t matter how big the capacitor is. Actually, the bigger the capacitor, the bigger the problems you will encounter. Bigger capacitors have longer charging times and draw more current from your alternator for longer periods of time. During this process, your amplifiers are starving for power because their voltage has dropped. And when their voltage drops, so does their current.

Q: Why? How could this happen? I thought a capacitor is supposed to prevent this!

A: No! 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, etc. Now that you 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 device: the 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 (i.e. 70A – 120A), the addition of a capacitor will do nothing more than add more stress to an already stressed 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. Hence 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 advertized!

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!

Remember: The accessories (i.e. 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 pulled by the car’s accessories due to the path of least resistance and the buffering action of the capacitor. And this happens because the capacitor’s voltage drops when 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, less available current is supplied to the amplifiers than is demanded; again, because there is a limited amount 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; thus 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. Hence, 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:

  1. The capacitor’s current is very limited in duration (time), magnitude (amperes) and thus 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 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 is known to 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 “voltage wall” and onto the amplifiers. The pernicious capacitor steals current from the amplifiers and gives it to the accessories (lights, ignition, A/C, defogger, etc). It acts like a fluctuating “voltage wall” that prevents a portion of the alternator’s otherwise 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.

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