Second Battery for Accesories

I have read some of the posts, and my questions has kinda been answered... but I want to make sure.

I was wondering if you can run a second battey for all accessory items: Stereo, Amps, Accessory Lights, etc.  Now, I know you can... but what about running the stereo without the car being on.  If this is possible, is there a particular size battery I should use, and can it be smaller than the one that powers the car?

Sorry for all the questions.  I have an MR2 Spyder and my space is VERY limited.  I'm just trying to find a way to run the stereo when the car is both on and off without wasting the main car battery.  Thanks!

Well, I have a 2nd gen MR2 and I may know a thing or two as well. The only thing that adding a second battery to your vehicle will do is give you a longer run time with the engine off. That said though, it must be charged. If you want to do this, it will require a good gel cell battery like an Optima and a Multi Battery Regulator. You cannot and should not just add a second battery and wire it up to the first one. What the regulator will do is ensure that the primary battery will always have enough charge to either start or operate the vehicle. Excess power will then be channeled to the second battery. An Optima battery or similar Gel Cell battery is the best option for you. It does not have a gas discharge problem, nor acids that can leak out, thus it can be mounted in any direction in any place. With a conventional battery it must be mounted right side up and vented to the exterior of the vehicle because of gas discharge. It is not a cheap battery but it is what you will need.Top Secret, I can tell you but then my wife will kill me.