Copied from earlier post below by Shizzzon. quick answer- A cap will discharge EVERYTHING in it's bank when a rush of current is requested... A battery will compensate for the current requested when the alternator can't or the regulator doesn't react fast enough. If you could.. try and imagine your battery going completely dead then back to recharging every other few seconds?!?! You can't imagine that at all.. So, you got your alt, a big battery bank and a big ass cap- The cap will ALWAYS be getting drained and recharged all the time... What's recharging it? The alternator... When it's doing that, it's providing less power to the audio system which results in more voltage drop because the battery(ies) must compensate more... Now, when the batts are being used more... every split second when when there is no demand for current, the alt must charge the batts back up too!!! Keep playin music like that... your voltage will continue to drop and drop.... That's why caps are BAD! They were even tested in a comp setup for power draw... IF a cap can disperse current quicker.. it should provide the amp with more power... A 600w amp was tested.. using a cap suited for the amp, the amp only outputted an additional 3w! 3 WATTS! Wow.... at that rate... u can just give me your money if u plan on blowing it.. Caps are useless period. After reading the above post I have come to the conclusion that you don't know what you are talking about and I urge others to take your thoughts on this issue with a grain of salt. I'm not here to question the merits of batteries or caps, they really are completely different animals and have their own place in a car audio system. Just pointing out your flawed reasoning and obvious lack of knowledge regarding electronics/electricity.