how would i run dual batteries one for electronics and one for the car? do i need a dual output alt? how many amps do i need 150? how would i do this?

The cleanest way to do it would be to buy a diode isolator like the ones used in Motor Homes. You can get them from JC Whitney. They allow both batteries to charge from a single alternator, but they isolate the battery loads so even if you run the aux battery dead with the accessories, the main battery is still at full power for starting and running the engine. The size of the alternator you need would be determined by the load you intend to put on the batteries. I have a dual setup in my pickup, and I have no problems keeping both batteries up with a 65 amp alternator, even at extreme cold temperatures when a battery cannot charge until you heat it up for a while first with the alternator.

First question is why are you thinking of running dual batteries? If it's because your system is pulling more power than your current setup can deliver (i.e. your lights are flickering like crazy), then having a second battery will only compound the problem. Why? A battery's job is to start the car and provide power in the event the alternator dies. Period. Once the car's been started, it's main job is done. So it kicks back and gets recharged by the alternator. So now it's also a draw on the system! Having multiple batteries only compounds this problem. If you're looking for a second battery because you're looking to run the system for longer periods with the engine off, then yes; a second battery would help. As Gary S said, use a battery isolator.

yes i want to run a system wth car off at shows etc.