Taylors right, not very subtle, but right.
Power caps were a craze that started because people thought it would smooth out the supply from the battery on a car.
Doah !! Battery, Capacitor,......er aren't they the same thing ?????
To be serious, your alternator wont be saved by anything if your overworking it by drawing too much current out of the battery.
What would be worth doing, is fitting a higher capacity battery, or even just a new one, as the time from flat to charged, deteriorates with age as the plates become coated.
The basic problem is, the battery/alternator set up on a car is not designed to cater for megawatts of music and you cant get something for nothing out of a charging system.