Dude, technically, they work. On large draws from the system (i.e. long bass hits), the cap "speeds up" the flow to the amps. But, after that long bass hit, the cap needs to be recharged. Therefore, the alternator has to charge a battery, and a cap.

It's a sales technique that most people at best buy use, saying that you will kill your alternator without one, which is simply untrue.

I ran 1500W (actual watts, not "peak" shit) in my 1994 Protege, that came with a 65A alt. I had 4ga running as my MAIN power line. I did this for about 2 years. All I did was re-ground my battery with 2ga wire.

Result?

No dimming whatsoever. Nothing. No electrical/alternator problems.

Caps are BS for daily use. However, when doing SPL burps, they can help, but most associations ban the use of them.