if you have sub's, amp's, dvd players, ps2, cd player, monitors, gps, phone and i-pod chargers, neons, reversing cameras and other electronics should another battery be added and connected to the alternater to run these items for a short time eg 30min when the car is not running?
If you're using all this equipment while the engine is running, then a second battery won't help you. The alternator is the primary power source while the car is running. No power is drawn from any battery until the alternator is already overloaded. Until then, additional batteries just add to the load on the alternator.
If you're having power issues then you may need to upgrade the alternator. If you use the electronics for long periods of time with the engine off, then you may want to install a second battery with a relay or isolator so that the starting battery won't be drained.
Much of the equipment you listed---LCD monitors, GPS, phone and iPod chargers, backup cameras---require very little power to operate. The amplifiers are the devices that require the most power, and it varies depending on how loud you play them. If you're not trying for high SPL, then your stock charging system might be just fine.
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