Installing multiple batteries can help preserve the life of your car's starter battery by giving your car's electrical devices another power source so they don't drain the primary battery. A marine or other deep-cycle battery is best to use for the second battery because it has a greater discharge capacity. If you already have the first battery mounted within its battery tray in the engine compartment, you just need to know where to place the second battery and how to wire it in the car.
Choose a location to install the second battery within the car. The trunk is usually the best location, as placing it in the engine compartment with the first battery will likely require a major modification to the compartment.
Install a battery tray into the trunk to store the battery in, screwing it into the floor of the trunk. Drill a second small hole into the trunk floor (making sure you don't hit anything such as the fuel tank) so the battery cables can travel out and along the chassis to the first battery in the engine.
Connect a battery isolator to the car's alternator. Connect the isolator to the positive terminals on both batteries, using separate terminals on the isolator. Secure the wire for the second battery to the chassis, using fasteners.
Make sure the positive terminal on the first "starter" battery is connected to the starter and ignition switch. Connecting the stock positive battery cable will take care of this. Connecting the car's stock negative cable will ground the first battery.
Wire the second battery's positive terminal to whatever source it's sending power to, such as the subwoofer, DVD player or stereo. Ground the second battery by connecting the negative terminal to a good metal ground like a spot on the chassis, securing the wire lead with the screw.
