Thank you Audio Proffessor JLATD, That was really helpful, I learned a lot from that. so in my case, i have 2 800w subs, running off of a 1000w amp; I will be putting in another 1000w amp pretty soon, in my case should I put a capacitor in also? or would I need another battery to just run the system? or what would I need to do for this to not fry my car?
I really don't want to turn this into an argument because misconceptions on this topic run deep. I am not asking anyone to buy it, just try it on. Remember that I am speaking from the book of Ohm's Law which does not deal in opinion.
All electrical energy in the car ultimately comes from the alternator. Batteries and capacitors are storage devices that need to be recharged. When current demand exceeds the alternator's output, the battery steps in to provide the necessary current. The battery can help only until it's depleted. A "dead" battery may still read 9V.
Bottom line is if we try to bolster the charging system which is not up to the task with batteries, you actually reduce the available energy from the alternator to the system. You will increase the reserve time by perhaps 15 -20 mins per battery, but the system will still hit the wall. When it does, the alternator's duty is now split up even more (has to charge all these extra batteries now).
The number 1 best thing to do would be upgrade your alternator. Extra batteries only really make sense when you have a show car that will play with the engine off.
Capacitors are basically useless because they can never recharge to a voltage higher than what is available. As far as the way they operate, the will do much better on the high frequency amplifiers than they will do on bass amps (spits in the face of conventional wisdom). Save your money for the new alternator fund.
I am not sure what kind of amplifier's you have, but I assume they are unregulated (most are). These type of amplifiers will simply reduce output as voltage drops. This is good news and bad news at the same time. The good news is: the amplifier is very charging system friendly. The bad news is: you won't get the power you paid for. If the power spec number is 14.4V, you are already in the hole 300 Watts at true in-car voltage (12.5V) so you don't want to loose anymore.
If you add another 1000 Watt (14.4V) amplifier you will have a total of 1400 Watts available. If you are an abusive listener, you will need 112 amps of current to produce this kind of power. If you have the same situation jenn has above (stock alt), these two amps cannot produce more AVERAGE power than 360 Watts. This means adding the second amplifier will not make your system any louder than it is now. In fact, if jenn had the same woofer displacement as you do, jenn's system would be just as loud with her 600 Watt amplifier as your's with two 1000 Watt amplifiers.
So if you do everything that is necessary (upgrade power wire, new alternator) your subwoofer system will be +3dB louder than it is now (assuming you have enough current now). If you do not upgrade the alternator, it will look cooler, but there will be no more average output.