zakimak

While a well designed system should never need a big capacitor, it's use is really dependent on the system and should not be frowned upon so readily. If your lights dims only on bass notes, and all is good otherwise, adding a capacitor would help. The cost for one and ease of install is worth a try. The best way to go is always an alternator that is capable of supply the necessary amperage to the system. That said, it is not always practical. Think of the capacitor is a fast filling fast replenish reservoir. Adding one will allow the amps to draw momentary power. It cannot supply sustained power. Your alternator provide that. Because music is transient in nature, power draw often spike, this then come into play when your alternator cannot supply the power but with the capacitor in place, the headroom can be compensated.

i.e.

Alternator supplying constant 70 amps @ idle
Amps requires on transient 65 amps average and 85 amps @ 25 ms

The capacitor would help with the 85 amps spike.

Remember a battery is also a crude capacitor and is another load when it is charging. It has higher capacity and reserve but at a lower discharge rate vs a true Cap. Adding a second battery will increase your reserve listening but also increase your load on the alternator. The bottom line is adding a second battery through an isolator will prevent your system from the draining the main battery, thus allowing you to start your car. It does not appreciatively solve the dimming of your lights.