So, you just installed your new stereo, but you notice your lights dim when the bass hits?
You're buying parts to build a system and youre worried that your lights will dim because (insert name here) said that you need one?
If you answered yes to either of these questions, a capacitor is not the answer to your problems.
A capacitor acts like a small battery to filter small ripples in your electrical system, it discharges when the input voltage drops below the charged level of the capacitor to keep a constant voltage.
Now you're probably wondering why they wont fix your problem if they do what I just said, right?
Well, here is your answer, a capacitor simply will not support any electrical components placing that much demand on your electrical system. Sure you could get a huge capacitor. The price of one of a capacitor large enough to adequately support that substantial of an electrical system just isn't worth the difference, and you're still not fixing your underlying problem, an inadequate charging system. Once the capacitor is discharged from being over extended from being used improperly, it becomes even more of a strain on your already limping electrical system by trying to recharge.
Now you ask, if a capacitor wont fix my problem, then what will?
Well, there are three steps that you can do to combat your inadequate charging system. Steps 2 and 3 are interchangeable, I would recommend them in the listed order, but it isn't as important as the first.
Step one: upgrade your big three wiring, these wires are:
1. Alternator to Battery (+)
2. Frame to Battery(-)
3. Engine to Frame or Battery (-)
See the following link for more detail.
Big Three Upgrade: How To
Step Two: Higher Output Alternator.
Simply replace your factory alternator with an alternator that put out more amperage. The most common and cost effective is the AD244 from the newer full size trucks.
See the following link for more information about the swap.
AD244 Upgrade: How To
Step Three: Add Another Battery
Just like it says, add another battery, you should use at least the same size wire that you used for your big three, if you have any considerable length of wire to reach the new battery, you need to fuse the wire at each end to help prevent any electrical fires if the insulation should become compromised.
If you are dealing with batteries of different types and or ages, an isolator should also be used to prevent premature battery failure, but this will also cause a half volt drop across the isolator.
This should fix your problems for up to a moderately high powered system (2000-4000 RMS Watts) if it doesn't repeat steps 2 and 3 and necessary.
Now that you've read why you don't need a capacitor, now you can see what they are useful for.
You already know that capacitors filter ripples in your electrical current from the beginning of this tutorial.
There are sound quality benefits to using capacitors as a filter before your amps. The first benefit is if there is an immediate need for a high current draw such as a bass note with a very fast attack, the capacitor can discharge faster to supply the required current, in addition to the current that is already there from your battery and alternator to keep your amplifiers response as fast as it can be.
Secondly it helps keep voltage spikes from reaching your amplifiers.
In conclusion, unless you have extremely fine tuned hearing chances are, you will never miss a capacitor in your system, but, I will not discourage you from getting one, so long as you are using it for the right reasons, if you are trying to fix a voltage problem with it, you are simply trying to put a band aid on a bullet wound, but if you have a strong electrical system and are looking for something to smooth peaks and provide an additional kick if its needed, chances are a capacitor is what you are looking for.