I'm wanting to mount a 3rd battery somewhere along the chassis of my truck to wire all of my accessories to (winches, random switches, AV components, compressor, etc...) that is kept charged by the alternator, but does not power anything for the truck itself. Is this something that would be easy to do? If it can be done, I will probably use a Marine battery so that it can go on it's side, but can a Marine battery also be mounted upside-down? And If it can be done, what gauge wire do I need to use to hook it up?

 

Buy a battery isolator. This will let the alternator charge both your normal batteries and the 3rd battery, but keep the 3rd battery isolated. Buy a isolator that is close to the output of your alternator.

The wire feeding the 3rd battery does not need to be too big. For easy installation and to make sure it has enough capacity, I would run two 10 gauge wires side by side in parallel. I would put a 40 or 50 amp circuit breaker coming out of the isolator feeding the two 10 guage wires which will charge the 3rd battery.

Use a marine or deep cycle battery. A marine battery is a "inbetween" type battery, giving you some starting amps and being able to be discharged deeply on a regular basis. A true deep cycle battery would be even better and last longer. You are going to want to buy one of those sealed batteries that has a paste inside instead of liquid acid if you want to turn the battery to a weird position. If you re going to keep it upright, a regular lead acid deep cycle battery would work.

You are going to want to have a power distribution spot at this new battery to hook all your accessories. You can use a aux fuse box for the smaller stuff, and probably be able to buy some heavy duty fuses and lugs from the stereo store for the heavier stuff. It's good practice to fuse your new circuits incase something bad happens to the wiring.