wiring up 2nd battery

I had an old battery laying around for years and thought it was dead but recently i threw it on the charger for the hell of it and to my surprise it retained a good charge. Nothing better than saving a hundred bucks next time i need a battery! Anyway i have a purely offroad rig that doesnt see a lot of run time, but since i have this battery im thinking of putting it in the back and add it to the charging system. I do not want to wire it up so it can start the jeep, only be charged by it. Basically my question is - i need to run pos and neg wires to the spare battery and put some kind of switch inline to be able to turn the charging on/off, and that will be all I need? I know it will work so im not really sure why im asking, but just curious what everyone thinks/runs. All i want is a second battery that is charged in case the primary battery is low or dead so all i have to do it swap batteries to get the thing started