Well let's see. Your alternator is probably using around 80 amps for every accessory in the vehicle to run at the same time and fire the engine. Maybe 50-60 with all of that off. So you have roughly 100 amps to work with. Maybe 75 before the voltage begins to drop. Now, let's assume your 1800 watt amps actually do rated power. Let's also assume they are 80% efficient (that's pretty good because I'm also guessing they do full power at 1 ohm, and amps aren't very efficient at 1 ohm) This means that at 14.4v each amp is going to draw 150 amps of current. So to make 3600 watts of power, at 14.4v @ 80% efficiency you're going to draw 300 amps of current. When your voltage drops, (it will) the current drawn does not go up to magically maintain 3600 watts of power output. When the voltage drops, so does the current drawn and thus power output. Also, when the voltage drops, the amps become less efficient.
You will need a good battery or two. I would suggest a larger alternator and extensive work on your grounds. You are not going to keep your voltage up at 14v. Forget about it. Capcells are marketing gimmicks.
