With the engine off, your system runs off the battery, and the battery alone. If you want to be able to play your system for a long time with the engine off, you only hope is to add more batteries or get a power supply, and plug it into the a wall outlet. Wall outlets are not always around when you want to "hang out" though....

Now, if you add 3-4 batteries, you still have to (highly suggested) seperate them from your main car battery. This way, if you drain the 3-4 stereo batteries, you can still start the car to go home because you have (seperated) isolated then from the car battery. Also, you will now be asking your alternator to charge 1 fresh batery, and 3-4 dead batteries. This will create a strain on the alternator, so be careful there too. You also might want to look into upgrading the alternator at some point.