How to Add a Second Battery to a Vehicle

by Chris Moore

With the numerous electronic devices installed in vehicles, the one starter battery may not be enough to handle the load. You can prolong the life of your vehicle's starter battery by adding a second battery that can power one or more of the electronic devices. The second battery can simply be mounted inside the vehicle itself, but it needs to be connected to the alternator to keep it from draining.

Step 1

Pick a location inside the car to mount the second battery. While the engine compartment is often the first consideration, this often requires modifying the compartment to make room for it. The trunk is a better location, especially if you're using it to power something in the trunk like a subwoofer.

Step 2

Mount a battery tray in the chosen location, using a tray that fits the second battery's size. Screw the tray into the surface (like the floor of the trunk), then drill a hole nearby to route the cables through. Make sure you don't hit any other components like the fuel tank when drilling and screwing.

Step 3

Unsnap both cables from the starter battery to shut off the car's power, disconnecting the negative cable first. Disconnect the wire that connects the alternator to the starter battery (this wire is part of the car's positive battery cable) by unbolting it or cutting it off at the alternator end.

Step 4

Install a battery isolator (a switch box for the batteries) into the vehicle. Strip away part of the insulation from the disconnected alternator wire with a crimping tool and connect the bare lead to one of the isolator's side terminals by loosening the terminal and retightening it with the wire lead under it.

Step 5

Connect the alternator to the isolator's central terminal (usually labeled "A") using an extra wire. Mount one bare lead of the wire under the isolator terminal and then bolt the other lead to the alternator. Connect the isolator's third terminal to another longer wire that will route all the way along the chassis to the second battery in the trunk.

Step 6

Connect the longer wire to your second battery's positive terminal using the same procedure as with the other terminals. Connect that battery terminal to the electrical connector of the device it will power (stereo, subwoofer, etc.). This can involve several possible methods like crimping or screwing the wire to a connector terminal.

Step 7

Secure the wires to the chassis by bolting or screwing wire clamps to the chassis. Make sure you have a clear path along the chassis where the wires and clamps won't be in the way of any lines, manifolds or other components. An exact clear path will vary depending on the vehicle.

Step 8

Reconnect the two car battery cables to the starter battery, starting with the positive cable. Ground the second battery by fastening a wire for the negative terminal to a metal ground like the chassis with a screw.

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