Originally Posted by wrx-killer

^^^^^^ I have to strongly disagree to how you wired up that second battery. The way you have it will kill the alternator in heartbeat or a day or so. I see this all the time. Its correct because its parallel but not because of the load. If you are going to run a second battery you are going to need a battery isolater. Like this one http://tsunamionline.com/product.php?id=480 .
This unit keeps each battery seperated so they can do there job right. Your car battery will operate the car and the second battery will operate the soundsystem. They will not cross talk. So if you drain the soundsystem battery it will not drain the car battery. And it will charge the weak battery first. This keeps the alternator cool and under less pressure. If you would of did it the other way you would need to have two of the same batteries. With the same cca, and they would have to be matched. Which means they would have to be of the same condition ie. age or close to the same voltage. This is because the alternator might charge one battery to much when the other is not full charge. Then your full charge battery will bust or fail. Adding a high amp alternator is really not going to solve the problem because first most don't provide power at idel or hold 12-14 volts at idel. One way you can fix that problem is dropping to a smaller pulley or have the company wrapp it so it puts out power at idel. Do not try to run a soundsystem of the alternator. It just plain won't work and you will burn it up. If you go with a alternator make sure you go with a better battery .

Originally Posted by wrx-killer

This is because the alternator might charge one battery to much when the other is not full charge. Then your full charge battery will bust or fail.