i get this question a lot at my shop. then i ask why?

a second battery will only be as good as a the alternator charging it. if you have such a large amperage draw (lights, winch, sound system etc.) that you think you need a second battery for it, but haven't got an alternator strong enough to keep one battery charge, you will just end up with a two dead batteries instead of one. i highly recommend a gel cell battery like an optima, and if you are going to buy one, spend the extra coin on the blue top deep cycle, it makes a world of difference over the standard red top.

the best way to go about this is to find out the amperage of your alternator and work within those parameters. every little bit helps. i've got LED interior lights in place of my factory dome lights. this means i have more light throughout the rig & also a fraction of the amp draw. i have switched my rock lights out from halogen to LED and can now turn my jeep off and not have to worry about a parasitic draw when out on the trail, or wasting gas leaving it running for the light.

the only benefit IMHO for having a second battery is to have an isolator that would allow that battery to change but not to discharge or be introduced into the system unless you switch it "ON". this is good when you have inadvertantly drained your battery by leaving the lights on. you will still have a second battery to start the vehicle and keep it running until your primary recharges.