Car audio, voltage drop

[ETERNITY]

I have installed a high end car audio system and I am not able to enjoy the car audio because it cuts off when I increase the volume

The internal and external lights dim and then the whole system shuts down, not just the sub and amp .
This Happens when I am stationary, when I am driving the system performs cuts on and off and does not play to its full potential. Sub cuts off and all my sounds distort.

I have been told I need a slave battery.

Can anyone clarify what the best way of dealing with this is so I enough voltage to be able to run the car audio full time