A capacitor WILL:-Act as a filter for AC ripple effect caused by the imperfect rectification of the AC current produced by an alternator, and rectified by a DC voltage regulator.
-Smooth the demand curve for current on the alternator, thus extending (theoretically at least) the alternator's lifespan, and, on a not so good note, this can also help to mask the symptoms of an insufficient alternator by smoothing these spikes that also cause the more obvious signs of a weak charging system, like dimming headlights when the bass hits.
-Reduce propagation delays in current supply for brief, spiked demands by the amplifier from the amplifier, and response to this by the electrical system. Amplifiers have to provide a very dynamic and quick response many times. A capacitor can assist in this if the rest of the charging system is up to par.
A capacitor will NOT:
-make your system magically sound 10 times better.Many people believe that a capacitor adds NO real benefit to an audio system, and this is why you never see before and after demonstrations, or factory capacitor company vehicles at car shows.
-Replace the need for a larger, high-output alternator and/or a deep-cycle battery or batteries.
If your electrical system is inadequate, the ONLY way to fix this, and again I repeat, the ONLY WAY to fix this, is to replace the alternator if your voltage rails are sagging to below 12 volts while the car is running. This is the SOLE source of electrical current for your car when the motor is running. The car's battery is in parallel with the alternator, and while the battery will help to stablize voltage at 12VDC, the alternator puts out a higher voltage, and if your voltage rails are dropping to 12 volts, you're already overdrawing the alternator's capacities, and if your voltage rails sag to below 12 volts then you're also going beyond the abilities of the battery or batteries to stablize your voltages. Both of these symptoms will result in battery and alternator damage.
A capacitor only masks these symptoms, in much the same way as regulating voltage to your headlights so they won't dim as bass hits, when voltages can fluctuate between 14.4VDC and 12VDC.
When the motor is turned off, the battery then becomes your source of electricity.
When the battery is run down, and when the capacitor(s) is/are depleted, the alternator has to work even harder in order to supply current to the car, the audio system, and also to recharge the capacitor(s) (which deplete more quickly) than they recharge) as well as recharge the car's battery(ies).
If you plan to use the audio system for prolonged periods at high volumes with the car turned off (such as for SPL events) you'll want to use isolated deep cycle batteries dedicated to the audio system to avoid damage to the batteries, and damage to the speakers and equipment from clipping.
So by adding a capacitor to try taking the place of a high-output alternator, you are actually causing more work for your alternator, and causing even more damage to that stock alternator.
A capacitor does have it's uses, but it is not a magical fix for a lacking electrical system.
Another item I'd like to touch on is the more recent advent of the high capacity capacitors (25, 50, 100+ Farad capacitors) and so-called "BattCaps."
These types of capacitors have extremely high ESR values which makes them very bad choices for the uses mentioned above. They are usually carbon-film based capacitors in order to get such high values, as opposed to the much lower ESR electrolytic capacitors you tend to find in teh 20 volt 1Fd type cylinder style units.
End result is avoid anything like the 100Fd capacitors like a plague.
