This question has been asked and answered about a million times already, but I'll go ahead and help you out. An extra battery wont really do you much good unless you have your stereo cranked while the car is off. Now, If you were asking if hooking your stereo directly up to another battery was a good idea, I would have to say no there too. The battery stays charged with a little help form the alternator, without the alternator hooked up to the battery it will just go dead pretty quickly.