But when the cap gets discharged it gets power from the battery in order to recharge. So in theory, you are draining your battery as much because your alternator is also drawing power at the same time. But of course this is all assuming he has a sufficient gauged wire.
Ive heard alot of people say upgrading the big 3 would give you the same if not better results than a cap. I would definately upgrade the big 3 in any car with a system in it. Its cheap, it doesnt take long to install, and improving the vehicles stock grounding system cant hurt. All in all, upgrading the big 3 would probably cost you no more than $20 if you get some real nice wire. $10 may even be enough.
From The12Volt.com:
--They won't help prevent alternator failure.
--They can't store very much charge, so they don't help with longer musical peaks, like sustained bass notes.
--They don't make the bass louder: higher voltage isn't the same as higher gain.
--They don't help if an alternator is just too small, and is continuously overloaded.
--They won't necessarily reduce dimming much, because they still need to be recharged.
--High-farad capacitors typically have a high internal resistance which makes them useless. In general, it's better to have five 1-farad capacitors than one 5-farad capacitor.