Installing an auxiliary battery in your car truck or boat is good insurance against being stranded by a dead battery. Connecting the second battery directly to the main battery is possible, but is not the best idea, because both batteries could be drained at the same time. A much better, and less expensive idea, is to install a battery selector switch, which will likely take less than an hour to install using hand tools.
Select a location to mount the battery tray or box. This can be anywhere in the vehicle, but as close to the main battery as possible is best. Mount the tray using bolts or self tapping screws.
Disconnect the negative terminal of the main battery.
Disconnect the positive cable of the main battery at the starter solenoid. This may be located on the inner fender near the battery or actually on the starter.
Connect the end of the battery cable removed from the starter solenoid to terminal one of the battery selector switch using one of the nuts supplied with the switch.
Connect a battery cable from the positive post of the second battery to terminal two of the battery selector switch.
Connect the third terminal of the battery selector switch to the starter solenoid terminal where the positive cable from the primary battery was originally connected.
Connect a negative battery cable from the negative post of the second battery to a good ground such as the vehicle frame, engine or transmission.
Reconnect the negative battery cable to the primary battery.
