How to Wire a Second Car Battery

Written by stephen benham
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How to Wire a Second Car Battery
Wiring a second car battery is easy. (vintage car chrome engine image by Leticia Wilson from Fotolia.com)

There are two methods of wiring a second car battery. The first method is parallel: this can double the endurance while the voltage remains unaltered. The second method is series: this maintains the same endurance but will double the voltage. Only wire a second car battery in series if you are certain your car's voltage matches the combined output of the two batteries; otherwise, you will damage your car's electrical system, and the fuses will blow. Wiring a second car battery, whether parallel or series, is easy.

Skill level:
Easy

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Things you need

  • Car battery cables
  • Second car battery

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Instructions

  1. 1

    Disconnect the two battery cables from your car with a wrench before you wire a second car battery.

  2. 2

    Position the second car battery so it's firmly fixed and near the existing battery. Use one of the existing battery cables and a new battery cable and connect one end of each cable to the negative (-) terminal of the existing battery.

  3. 3

    Connect the existing battery cable and a new second cable to the positive (+) terminal of the original battery. Ensure the loose ends of the battery cables do not touch each other or your car.

  4. 4

    Attach the other end of the new battery cable, already connected to the negative (-) terminal of the existing battery, to the negative (-) terminal of the second battery. Connect the other end of the second new battery cable connected to the positive (+) terminal of the existing battery to the positive (+) terminal of the second car battery.

  5. 5

    Reconnect the original battery cable, connected to the negative (-) terminal of the existing battery, on the car that was disconnected in Step 1.

  6. 6

    Reattach the second original battery cable, now connected to the positive (+) terminal of the second battery, to the car that was disconnected in Step 1. You have wired a second car battery in parallel.

  1. 1

    Disconnect both battery cables from your car before you wire a second car battery.

  2. 2

    Verify that your car is set for the voltage created by adding two car batteries. If you are adding two 12-volt batteries in series, the output will be 24 volts.

  3. 3

    Connect a new battery cable to the negative (-) terminal of the existing battery and attach the other end to the positive (+) terminal of the second car battery.

  4. 4

    Attach the original negative (-) battery cable to the negative (-) terminal of the second car battery. Reconnect the original positive (+) battery cable to the positive (+) terminal of the existing battery.

  5. 5

    Reconnect the battery cables connected to the second battery's negative (-) terminal and the existing battery's positive (+) terminal to your car. Your car now has a second battery wired in series.

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