A capacitor put more of a load on your alternator. as any addition in the electrical system, but saves a small amount of energy (fractions of a second worth) for use during deep bass hits. Also, they help stiffen your voltage rails to provide a more even flow of energy. If you need a capacitor to fix your dimming or what not, you might as just spend the money on an alternator. The one you are using will eventually burn out if it's being overrun continuously.
There used as a quick soruce of power when a note or notes is needed for an extra "kick" so to say. A capacitor charges and discharges very rapidly, however your Battery must recharge it quite often. An HO Alternator may be better idea though, depending on your situation.