Here is some interesting info about caps and what they do:
What is a "stiffening capacitor", and how does it work?
Stiffening Capacitor (note capitals) is a trademark of Autosound 2000. However, "stiffening capacitor" (note lowercase), as a generic term, refers to a large capacitor (several thousand microfarads or greater) placed in parallel with an amplifier. The purpose of doing so is to provide a sort of reserve power source from which the amplifier can rapidly draw power when it needs it (such as during a deep bass note). The electrical theory is that when the amplifier attempts to draw a large amount of current, not only will the battery be relatively slow to respond, but the voltage at the amplifier will be a little lower than the voltage at the battery itself (this is called line drop). A capacitor at the amplifier which is charged to the battery voltage will try to stabilize the voltage level at the amplifier, dumping current into the amplifier. Another way to think about it is that a capacitor in parallel with a load acts as a low pass filter See section 3.10 What is a crossover? Why would I need one? [JSC], and the voltage level dropping at the amplifier will appear as an AC waveform superimposed upon a DC "wave". The capacitor, then, will try to filter out this AC wave, leaving the pure DC which the amplifier requires.
Do I need a capacitor?
Before installation, it's often difficult to predict whether or not a capacitor will be beneficial to you. It's generally best to install the audio equipment prior to making the determination, so that you can address which symptoms need to be remedied and assess the severity of the symptoms. This will not only help you decide whether or not you need a capacitor, but also how much capacitance would be beneficial.
The most common symptom in need of added capacitance is headlight dimming (and sometimes dimming of the interior/dash lights). It's caused by a drop in system voltage associated with excessive current draw. While there may indeed be several loads drawing substantial amounts of current from the electrical system (eg. heat, AC, and so forth), it's usually the transient draws that best manifest themselves in noticeable dimming. This is partly because our visual systems are most sensitive to detecting rapidly changing intensity levels rather than steady absolute differences.
Once you've assessed whether or not the dimming is noticeable (and sufficiently annoying), you must decide whether a capacitor is warranted or if you'd be better served by upgrading the alternator. After initially having your alternator and battery checked out (some places will do this for free), the choice should be based on the severity of the dimming.
A commonly-used estimate for determining the appropriate size capacitor is 1F/kW (one farad per kilowatt). For example, a system running at 300W would need a 0.3F (or 300,000uF) capacitor. However, there are several variables at play here, including the capabilities of the vehicle's electrical system (which generally varies from idle to higher RPMs), the efficiency of the amplifiers, and the listening habits of the user (ie. the tone controls and the type of music). These factors should all be considered when making the determination. Moreover, the voltage drop can be so severe that added capacitance is nothing more than a band-aid. That is, even several Farads of capacitance would not be able to sustain the voltage for as long as the drop persists. This is when an alternator upgrade may be in order.
Can I just upgrade my headlight wiring instead?
Although headlight wiring upgrades can often be beneficial for achieving a higher steady-state illumination, it will not improve the dimming situation. Since the headlights are not the cause of the voltage fluctuations that are producing the dimming, upgrading the wiring will not fix the problem. The voltage fluctuation is present at the battery terminals, so it will be transmitted to the headlights regardless of how the headlights are wired. If you think of the fluctuation as an AC signal, then it becomes readily apparent that this circuit can be represented by an AC signal in a voltage divider. Decreasing the resistance in series with the load by upgrading the headlight wiring actually serves to slightly enhance the AC signal at the headlight's terminals. In other words, the dimming effect could become even worse by upgrading the headlight wiring!
Will the dimming go away if I upgrade the amplifier power/ground wiring?
A common myth in the car audio community is that upgrading the power or ground wire to the amplifier will result in the amplifier drawing less current and therefore decreasing the voltage fluctuation. While the logic is sound, the premise is not. Most amplifiers on the market have semi-regulated supplies which don't maintain a steady power output at a range of supply voltages. This is reflected in the power ratings provided by many manufacturers; some provide ratings for their amplifiers at two different voltages, and the lower voltage almost always causes the amp to deliver less power. In general, the difference in power output tends to correspond well with the supply voltage such that the current draw remains roughly constant (assuming somewhat similar efficiency). Consequently, upgrading the power/ground wiring, which serves to increase the voltage at the amplifier's terminals, will not reduce headlight dimming.
What do I look for when buying a capacitor?
The single most important attribute is the capacitance value (expressed in Farads). Put simply, more is better. Another important consideration is to make sure the maximum voltage rating of the capacitor safely exceeds the operating voltage of your vehicle's electrical system. In addition, ESR and ESL values may be provided with some capacitors to essentially indicate the amount of voltage drop that occurs when a capacitor is delivering current. Smaller values are better in this regard.
How do I install a capacitor?
If you conclude that your best course of action is to install a capacitor, it should be installed in parallel with the amplifier and, generally speaking, should be wired with approximately the same gauge wire used for a single amplifier (usually 8 ga. is sufficient even for rather large capacitors).
Before permanently installing it, it must be charged. Failure to do so could lead to blown fuses and lots of sparks! Some capacitors come with charging resistors. If yours does not, you can simply buy an automotive bulb and wire it in series with the capacitor's + lead while the capacitor is grounded. The bulb will continue to dim until the capacitor is fully charged. Once the capacitor is charged, it should be treated as you would a car battery; caution must be used to be sure not to short the terminals.
The final step is to permanently install it into the car. There's been much debate about where to install the capacitor. It's been argued that the placement is important because it requires shorter wire lengths. While this is true, there has never been any evidence supporting the notion that it should be installed as close (electrically) to the amplifier as possible. In fact, electrical theory demonstrates that it's more effective at quenching the dimming effects by installing it as close to the device exhibiting the symptom (ie. the headlights) rather than the device that's drawing the bulk of the current (ie. the amplifiers). However, the benefit to doing so is negligible. Therefore, hooking it directly to the battery, the amplifier terminals, or the distribution block are equally valid solutions as long as the mounting location is safe, the wire lengths are reasonably short, and there's an adequate ground present.
I have more than one amp in my audio system. Which one should I have the capacitor run?
The amplifiers are all connected in one way or another to the battery. In fact, unless you're running separate power wires to each amplifier all the way from the battery, they're usually connected at a more proximal site (a distribution block, for example). The effects of the capacitor are felt by the entire electrical system, including the amplifiers. Therefore, you cannot selectively dedicate a capacitor to a specific amplifier.
Will my bass response improve by adding a capacitor?
A capacitor serves to smooth the voltage fluctuations associated with transient current draw. As a result, the supply voltage presented to the amp during peak demands tends to be slightly higher than without the capacitor. For most amplifiers, this will increase the power output of the amplifier during transients. The degree to which it increases, however, typically leads to an inaudible improvement.
To illustrate, if you consider an amplifier that delivers 100 watts at 14v and 80 watts at 12v (these numbers are somewhat typical), the difference in output from the speaker will be at best 1 dB when the supply voltage fluctuates from 14v to 12v. However, when you take into account the fact that no practical amount of capacitance can completely eliminate this voltage drop during transients, the difference in output becomes even less pronounced. Further, if you take into account other factors such as loudspeaker power compression (discussed elsewhere in the FAQ), the equivalent series impedance of the capacitor, the length of the transient, and the human's decreased ability to perceive differences in intensity for shorter intervals, this difference in output becomes negligible.
To say they do nothing is incorrect..
