How to Hook Two Car Batteries Together

There are two reasons for hooking up two batteries in your car. If you have several electronic devices in the car, you may need to connect the batteries separately so the second battery can power the devices without draining the first one's power. Or you can link two batteries to generate more power at once (like connecting two 6-volt batteries to get 12 volts). The wiring method is different for each of these.

Things You'll Need

• Car starter or deep-cycle batteries
• Battery cables

Instructions

Connecting Separate Batteries

1. Connect a battery isolator to your generator (in a car, this is the alternator). The isolator is a switch that can alternate the power between the two batteries.

2. Connect a terminal on the isolator to the positive terminal on one battery. Connect the second battery's positive terminal to another terminal on the isolator. If you need to use your own wire for this, use a new No. 8 GA wire.

3. Wire one battery's positive terminal to the starter and ignition switch, making this the "starter battery." Connect the second battery's positive terminal to the accessories you will use the extra battery for.

4. Secure the negative terminals of both batteries to a ground. The battery cable in the car supplies the ground for the starter battery. You must locate a good metal ground to connect a cable for the other battery. Placing the cable leads under a screw somewhere on the chassis will work.

Linking Two Batteries

1. Connect the red battery cable, which is connected to the alternator and ignition switch, to the positive terminal on one battery.

2. Attach the black battery cable, which is the grounding cable, to the negative terminal on the second battery.

3. Link the two batteries by connecting the first battery's negative terminal to the second battery's positive terminal.

References

• CustomBatteryCables.com