concurr with josh and natem; the cap will help, as would a higher amp alternator, but understand that playing your stereo with your engine off at either loud volume or for an extended period of time will not only drain your battery, but shorten your alternator's life (I learned the hard way!). If you have the space, you might even consider adding a second battery with a splitter(solenoid). That would help on both fronts. Also, most voltmeters don't have the high current reading capability that's required to do as natem described, so look at it first before you fry the poor thing--it should be able to meter up to at least 40 amps to be on the safe side, or just go to a shop where they have those handy inductive devices to read current. And finally, some amps that have regulated supplies can actually start ******* current like there's no tomorrow once the voltage drops below a certain level; kind of a snowball effect. Or, if you've got a class A amp(class D are the best at getting more power with less current draw).
