you would need a dual battery isolator DO NOT RUN A WIRE FROM THE ORIGINAL BATTERY TO THE SECOND ONE. If you go with an extra battery, let me suggest a high output alternator, two batteries needing a charge is a lot for a factory alternator to handle. Let me ask you this, there is a difference between why you would get a cap and an extra battery....are you trying to keep headlights from dimming, or are you going for extended play time from the system when the car is shut down. If you are just trying to maintain transient response for the woofer and give the amp a little more juice when it needs it, or just trying to keep headlights from dimming I would suggest a stiffening capacitor