i don't get the point of getting extra batteries if you don't have the power to supply them. so 1st thing i'd get is the alt and big 3. here's a site that i got from an avalanche website for alternaters.
high amp gm ford chrysler chrome alternator
It all comes down to how close your charging system is to keeping up with the demand placed on it when the stereo is turned up. Adding an extra batt does a couple main things.
One, it adds more storage capacity. As the stereo draws more power than the alt can produce, system voltage drops. Once system voltage reaches batt voltage levels (~12.8v), the alt is maxed out, and all power beyond that point is drawn from the batt(s). If this continues for a period of time, the batt(s) drains, and its voltage decreases. So obviously, more capacity is beneficial. More capacity means a longer period of allowable overdraw on the charging system before over all system voltage drops to a dangerous level and the amp goes into protect mode (or worse).
Second, usually a secondary batt is mounted in the rear, much closer to the amplifier(s). This means you now have a power 'source' relatively close to your main current drains. The spikes in current draw required to power the transient demands of music will draw mainly from the rear batt, as there is less line resistance between it and your amp than there is to the front batt. Less line-loss to the amplifier for better performance. There's also a lesser known/discussed advantage of a smaller required power cable running to the rear batt than if you ran from a front batt to the amps directly in the back. This is because the current passing thru the wire (front to rear) will only be to equalize voltage between the front and rear batts in the system. As I said, the rear batt will receive more drain from the rear mounted amps than will the front batt, so the demand from those peak current draws is 'smoothed out' in the form of voltage equalization between batts during periods of lesser current demand. Viola, smaller cable required for that long run from the front to rear of the vehicle.
And with all that being said, there is an important factor that must be considered: average current draw over time. Averaging out the peaks and valleys in current draw over time, a more generic statement can be made saying that if your current draw is greater than current supplied by the alt, you WILL run out of power at some point. How soon is factored by capacity in the system, and the difference between current drawn and current supplied.
Even your own personal listening habits factor into all this. //content.invisioncic.com/y282845/emoticons/ponder.gif.ec2527b23f1d287b60dcbba54f95edba.gif
I typed all this out to explain to you that there are no simple answers here. I do this because I feel the inevitable "how big of an alt do I need for this many watts?" question coming on. I hope Ive shown you that's impossible for us to tell you. And that to really plan out a charging system efficiently, instead of just guessing and hoping you dont damage anything if your voltage drops too drastically, or simply throwing money at the solution, you have to learn some of the fundamentals of how the system works instead of just asking some strangers for their advice.
