A cap or battery is not going to do much for that anyway. If anything you would need a higher amp output alternator. if the alternator isn't strong enough to keep the battery or capacitor charged properly neither will really help. If you see signs of voltage dropping in your car while playing your system then you would need to check into a new one. Check to see what the amplifiers amperage draw rating is also. Thi will help in determining what you are going to need.