Here is a quick explanation of all that stuff for ya, basically what the alternator does is generate the power, depending on your car can can have anywhere from a 35 amp alternator to probably a 120 amp alternator in your vehicle. Capacitors basically are stiffening capacitors, these even out the voltage flow since alternators arent designed to generate varying currents quickly, however capacitors do not compensate for a low output alternator in your system. A way to think about it is like this, a capacitor will be able to help with the quick current demands, however if your overall current demand over a certain amount of time exceed that of your alternators output you will be draining your battery and hurting your cars electrical system. Now people that put all sorts of batteries into their are are placing those in there to be able to run the system without the motor running and still be able to start the car (battery for starting and batteries for running the system). Depending on what car you need and the rms wattage of your system you may need a new alternator, capacitors are only usefull if your alternator can handle the average current demand of your system but not the quick hits.