There have been several posts on this so I am going to make this in hopes that people read it. First you must evaluate why you wish to run 2 batteries? Do you wish to run accessories without the rig running and not drain the battery? Do you want more starting power? There are a couple ways to hook up dual batteries. First you can just connect them in parallel. This doubles the available amperage while maintaining the normal 12v. To do this all you do is connect the pos and neg of both batteries together. It is recommended that both batteries are bought at the same time and are the same make or 1 may discharge the other. This is the simpleist and least expensive way to connect the batteries. Downsides are if one battery goes bad it will discharge the other in all probability killing both batteries. The second way is to use some sort of switch between the batteries. This has the advantage of separating the batteries so one will not discharge the other. The downside being you have to remember to turn the switch on and off and unless you like popping the hood every time you park will require running some heavy cable into your cab. Another option is the solenoid charging switch. If you have accessories that require a direct connection to the battery such as a winch, crane, liftgate, electric hydraulic dump bed or whatnot this is a cost effective system to use. Basically you run a line from the charging system through a ford style starting solenoid...and it seems the metal cased ones are good for this because they are rated constant duty...and to the battery. The switching side of the solenoid gets connected to the positive side of the ignition. So when you turn on the ignition to start your truck it closes the charging circuit and both batteries will be charged. While the vehicle is off the batteries will be isolated so you can run those accessories with confidence knowing that your starting battery will have juice. Another advantage to this system is that should your starting battery go dead for whatever reason you can use your jumpercables and jumpstart yourself. This setup also has the advantage of allowing you to run 2 types of batteries. so you can have a deep cycle as your aux battery and can run that pimp soundsystem at the next tailgate party for a long long time without the worry of fully discharging and damaging a starting battery. The final solution and the one most preferred is the isolator. Isolators can be found from $25 for the low end ones from JCwhitney to well over $100 from some online places. What an Isolator does is Allow your batteries to be connected to the same charging system at all times without requiring a switch and which will charge both batteries while still keeping them isolated from each other. This, like the solenoid isolation also allows the running of 2 different batteries. Now for an example in my Blazer I will be running a starting battery the will handle the running of the machine (starting, ignition, lights). I will have a totally separate and isolated circuit running of an auxilary deep cycle battery which will be for winch, aux lighting (fog lights, rock lights) power points both 12v and 110v through power inverters, and my stereo system and amplifiers. This way I can go to a 4 wheel party blast the tunes and run my spotlight without my rig running and still start my ride the next day.