capacitors are total crap, throw it out. And like everyone else has said, make sure you have done the big 3, bigger wire the better. Your battery is probably fine, who knows about the Alt cause you never said how many amps it puts out. Keep in mind that a stock alt was put in by the factory to handle a stock radio putting out maybe 25 watts, not 500. So your alt may help when your keeping the engine revving at 2K, but not so much when your at idle. And if your battery doesn't have the reserve to handle your amp then it's going to drain faster than your stock alt can charge your battery. Doing the Big 3 will help a little with this. If you still have voltage issues then adding a 2nd battery would be your answer, not a capacitor. You don't need to go and buy a huge battery for a 2nd one, but something cheap like a Kinetik HC600 or an HC800 would work well for your small system.