The capacitor is going to act like a small sponge so if you have a very hard Bass drop or some other type of short bursts of full power that capacitor will unload its juice instantly yes somebody can make a joke about that if they want. Anyways it's only goal is to keep unnecessary voltage fluctuation from happening but it does not store power for a very long period of time especially not if you run your system with the car off if it's between that or battery I would go with the extra battery because even though it does not discharge as fast as a capacitor it's a better sponge overall and you can play it with the car off