Some of the numerous advantages of having two batteries in your vehicle are the opportunity for reserve power, the ability to operate high-powered stereo systems, and being able to use electrical devices without turning on the engine. If you do decide to have a second battery, though, wire an isolator switch into the dual battery system so that one battery doesn't siphon power from the other--or you just might end up with two dead batteries.
Open the bonnet and boot. Disconnect the vehicle's battery by removing the negative (ground) cable with a wrench.
Mount the battery in your car, truck or SUV (the battery mounting location may vary). Place the battery box where you want it. Typically, the boot is the best mounting location. Place a drill bit into the drill motor chuck and drill through the mounting bracket of the new battery box and into the boot floor. Use the mounting hardware that came with the box and insert bolts through the holes you just drilled. Tighten them with a wrench.
Drill an additional hole beside the battery box to run the new positive battery cable through the boot floor. To avoid cutting the cable, place a rubber grommet in the hole.
Mount the isolator switch. Place the isolator switch in the engine compartment. The inside of a fender well or the radiator support works best. Place a screwdriver bit in the drill motor. With the isolator in place, run a self-tapping screw through a hole in the mounting bracket and into the fender well. Repeat this for the other mounting hole in the bracket.
Install the new battery in the newly mounted battery box in the boot and secure it with the included mounting hardware. Tighten it with a wrench. Run a piece of 4-gauge wire from the battery box in the boot through the rubber grommet and along the chassis into the engine bay stopping at the new isolator switch. Cut the wire with wire cutters. Secure the wire along the chassis with plastic zip ties.
Run a piece of the same wire from the original battery's positive post to the isolator. Cut this wire with wire cutters. Use the wire stripper to strip all four ends of the two new wires. Install terminal ends on stripped wire and crimp them in place with the wire crimpers.
Connect the positive (+) wiring. Place the new terminal end of the long positive wire in the boot onto the positive (+) post of the new battery. Tighten the bolt with a wrench. Place the terminal on the other end of that wire to the isolator switch post marked A (auxiliary). Tighten the bolt with a wrench. Place the terminal ends of the new short positive wire from the original battery to the isolator's M (main) post. Tighten both connections with a wrench.
Ground the system (-). Run a piece of 4-gauge wire from the negative post of the auxiliary battery box in the boot to the floor of the boot beside of the newly installed battery box. Strip both ends of the wire with wire strippers. Connect the terminal ends onto the freshly stripped wire with the wire crimpers. Connect one end of the new ground wire to the negative (-) post of the battery. Tighten this with a wrench.
Insert the screwdriver bit into the drill motor. Attach the other end of the new ground wire to the boot floor by running a self-tapping screw through the terminal end and into the boot floor. Attach the isolator's ground wire to the vehicle's chassis with a self-tapping screw run through the terminal end and into the chassis. Reattach the vehicle's original ground strap to the original battery and tighten the clamp with a wrench.
Power the isolator switch. Run a piece of 18-gauge wire from the "IGN" (ignition) post of the isolator switch to the "IGN" (ignition) terminal of the vehicles fuse box. Strip both ends of the wire with the wire stripper. Use a crimping pliers to attach a terminal end to each end of the wire. Connect one end of the wire to the isolator post. Tighten it with a wrench. Attach the other end of the wire to the fuse box by sliding the terminal end onto the lug. Use plastic zip ties to secure the new wire to existing wiring.
Close the bonnet and boot.
Use proper gauge wiring to install the system. This will insure the system will work as intended.
Never work on an electrical system without first disconnecting the battery. Failure to do this could result in electric short or fire.