there are many debates about caps vs batteries. a cap acts like a battery

Battery
-long charge time, has great charge for long periods of time

Capacitor (assume 1 farad)
- quick charge time about 2 seconds, charge is only good for short period of time

00exaccord as he mentioned it acts like a buffer. insted of your amp drawing the power from the batt, it pulls it off the cap just for that instantanious second when a deep bass note hits. It may prevent headlight dims for now, but what really gives power to your car? is it the battery? no, its the alternator. So really save up a little more and upgrade the alt. i think caps are useless unless you have a 500watt system and a 1 farad then your safe. anything as 1000watts + then doing a big 3, alt upgrade is best for the electrical system.