Not to resurect the smoldering coals of a dead flame thread, but I figured it was only fair to somewhat explain what a cap does and what situations it will and will not fix as this is the first working hit off of google when looking for what cap size you need.

A capacitor is very much like a fast-acting battery. Under no-load conditions, a cap will charge up to the supply voltage it is connected to. In the case of your car battery and alternator setup, it will charge to a little over 12V. If a sudden load is applied to the system, the battery and alternator don't "sit back while the cap does the work", they both still supply whatever energy they can. The cap simply supplies energy faster so it appears to take load off the system. Once the cap drains down (which happens --very-- quickly), it is useless until it recharges again. This recharge takes a finite (but small) amount of time and while it is charging, the cap is taxing your battery and/or alternator.

As was said before, a cap does not "add evergy to the system" a cap "buffers" or "filters" the energy in a system. In electronics terms, "a capacitor resists a change in voltage across its terminals".

So, as for what a cap can do to help you:

If you turn on your amp(s) and your lights dim and stay dim, a cap isn't going to help you. You have a constant draw that is more than your system can handle. You need to upgrade your alternator no matter what and your battery if you plan on using your stereo without the car running. Using your stereo in this situation is damaging your electrical system. You should cease usage of your system until upgrades have been completed.

If your electrical system doesn't react when you power on your amp(s), but the lights dim or something along those lines (In Dodge, the dash LEDs flash) during the particularly heavy bass hits, then a cap can help you out by charging during the down-cycles when your electrical system has some extra energy and providing the needed punch during the heavy hits when your electrical system can't afford the necessary power. However, if this is the situation you find yourself in, you're right on the borderline of what your electrical system can handle and you really should upgrade your alternator anyway. If you're planning on doing a lot of stereo thumping without the car running, you'll also need to upgrade your battery.

If your electrical system seems happy to power your amp(s), but your heaviest hits clip, then a cap may be able to help. If this is your situation, you probably have a reasonably high quality amp. What is happening is that the amp stops drawing power when the electrical system doesn't have it to give. Theoretically, the amp could cut out if the supply voltage drops below 11.5V or so. This idea sacrifices sound quality for the safety of your electrical system. As before, a cap will give you the punch you need for the hard hits so long as it has down-time to charge in between. Again, as before, you're right on the borderline of your electrical system's ability and you really should upgrade your alternator and possibly battery.

If your current electrical system seems happy to power your amp(s) even during the heaviest hits, this is when a cap can do you the most good. Under this condition, even though you probably won't hear the difference, the cap is still taking the brunt of the heavy load bass hits. What this accomplishes is saving your battery and alternator from experiencing the voltage drops caused by the bass hits. This voltage fluxuation is one of the major causes of premature death of your charging system components.

I hope this clears some stuff up. If anyone with a more intimate knowledge of power electronics notices any mistakes or misinformation, feel free to correct me.

Drive Safe