Caps store up power so the amps have more voltage to use when they need more then you can supply. A car battery wasn't meant for us to be putting huge radio's, subs, 5 amps, neon lights, and all the goodies that we so love to use. A better way to get more stored voltage is to just add a second battery, but sometimes the current isn't high enough...then you need a bigger alternator. Try the cap first though, it's the cheapest route.