the reason a cap generally is no good is because with the amount of power that it stores it is depleted after one large hit. so if there is another hit before it can recharge then it is pointless (and no its not like an instantanious recharge it takes a few seconds). the second battery you install in your trunk and hook your amp directly to it. then you wire the battery to your other one. with this when there is a large hit it will draw from the battery in the rear of th car and not your alternator. this 2nd battery is much harder to deplete than a cap because it can hold more amps.

and as for a bomb in the tunk........ thats what fusses are for!

 

the cap holds a charge even after it has been disconnected from the power source and if it get dislogged from mounts in a car accident and finds its way connectors down dead shorted to say the spare tire it will pop and throw the battey acid that is in side of it all over which could due more damage to your car or yourself. take one one day a take a screw driver and short it out (breifly)after you have tookin it out it gets really hot really fast i had one pop on me once when i work at ulimate electronics i chared it with a restiotor and let it sit on my work bench and the other install i was work with bump my bench and the cap fell in a metal trash can and we didn't know it till it (probaly a min shorted out)poped really loud and it short out some nasty goo every where and it moved the trash can about a foot and bented it.

if your only going to be running 400 true watts (RMS) then a cap nor a second battery will be needed.