Car Audio Capacitors Explained

How a Big Capacitor Can Feed the Beast That is Your Car Amp

flickering headlights may require a car audio capacitor
Flickering headlights are a classic sign that a powerful amp is creating a voltage drop, which can often be solved by installing a car audio capacitor. Image courtesy of eirikref, via Flickr (Creative Commons 2.0)

Car audio capacitors are one of the most commonly misunderstood car audio components, but they actually aren't that complicated. At a basic level, a capacitor is just an electronic component that can be used to store energy, sort of like a battery. In fact, the very first “batteries” were actually primitive capacitors that were made out of water-filled glass jars.

Unlike batteries, capacitors are designed to store and discharge electrical energy very rapidly if needed.

In car audio applications, this ability can be tapped in order to provide an on-demand source of power to an amplifier. Since the energy density of most capacitors is over a thousand times less than that of an alkaline battery, a car audio capacitor is typically much larger than those found in most consumer electronics.

On-Demand Power

Every capacitor consists of three basic components: two electrical conductors and a material known as a dielectric, which serves as an insulator between the conductors. When the conductors are supplied with a source of voltage, a static electric field is generated on the dielectric. This effectively stores electrical energy, which can then be released after the charging circuit is removed.

Capacitors have a variety of uses in electronics, from decoupling DC and AC signals to providing enough on-demand energy to start electric motors that require a lot of extra torque to get going.

Any given electronic device you own is almost certain to contain capacitors in addition to other components like transistors and resistors, and they are very commonly found in power supplies. Very large capacitors are used in devices like heart defibrillators and lasers due to their ability to discharge a great deal of power in a very short time.

Power Hungry Car Audio Systems

In car audio applications, a capacitor can be used to “feed” your hungry amp. The issue at hand is that an especially powerful amp may outstrip the power-generation capabilities of your car’s electrical system. When that occurs, you will typically see your headlights or dash lights dim during powerful bass notes.

The way a car audio capacitor works is that during normal operation, it is supplied with voltage from a car’s electrical system. That charges it up so that’s ready to go when it’s needed. If at some point the amplifier tries to draw more amperage than the electrical system is capable of putting out, resulting in a significant voltage drop at the amp, the capacitor will discharge. Since car audio capacitors are typically installed right next to the amp—or as close as possible—there is significantly less voltage voltage drop, and the whole electrical system is able to continue operating normally.

Since capacitors are able to charge and discharge very quickly, a car audio capacitor can be “refilled” whenever the power demands of the amp don’t require any extra juice.

Learn more about: Choosing an amplifier

How to Tell if You Need a Car Audio Capacitor

If you have a stock charging system and battery, and your amplifier is especially powerful, then you may end up needing a stiffening cap.

However, adding a capacitor to your car audio system won’t actually have any noticeable effects unless a few specific criteria are met. The main contributing factors that can lead to a car audio system needing a stiffening cap include:

  • an especially powerful amp
  • an underpowered alternator
  • a low-capacity battery
  • excessively long or low gauge amp power wire

The most important thing to look for is dimming headlights. If your headlights dim when you turn your radio up, or during especially loud bass notes, that’s a clue that your charging system isn’t quite up to snuff. However, a stiffening cap won’t necessarily solve the problem.

If the charging system is too underpowered, or the amp is too powerful, then installing a cap may not be enough.

Installing a Stiffening Cap

Installing a car audio capacitor isn’t especially difficult, although it can be dangerous. Since large caps are capable of storing and discharging a great deal of energy, they require a little more care in handling than other car audio components. When you buy a cap, it should come with detailed safety instructions, installation instructions, and either a light bulb or resistor that can be used to safely discharge it. If you’ve never installed a stiffening cap before, it’s vital to read and understand the safety and installation instructions before you get started.

In broadest strokes, installing a stiffening cap isn’t terribly complicated. Car audio capacitors should always be installed as close to the amp as possible, and they typically have either one terminal, two terminals, or a distribution block.

If a cap has two terminals, then the positive one should be hooked directly to the positive amp connection, and the negative one should be attached to chassis ground—preferably in the same place as your amp.

If it has a distribution block, then you can typically connect the positive battery lead directly to it. The positive terminal on the amplifier can then be connected to the distribution block as well.

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