How to Hook Up a Battery Isolator

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How to Hook Up a Battery Isolator
Installing an isolator switch can prevent you from having dead batteries. (Motorcycle key Switch image by Jim Mills from

The electrical system on your car has a very hard job. It must keep the battery charged fully for use in all kinds of weather. On hot summer days and frigid winter mornings it is asked to perform flawlessly and is not given any thanks when it does. It seems the only time most people think about their battery is when there is a problem. Some people install an extra battery in their vehicle to avoid the dreaded dead battery scenario. The problem is, without an isolator switch installed in the charging system, they usually end up with two dead batteries.

Skill level:
Moderately Easy

Things you need

  • Battery isolator switch
  • Drill motor
  • Drill bit
  • Wrench
  • Self-tapping screws
  • Screwdriver bit for drill motor
  • Wire stripper
  • Wire crimpers
  • Electrical terminals

Show MoreHide


  1. 1

    Disable the electrical system. Disconnect the vehicle's battery by removing the positive cable with a wrench.

  2. 2

    Mount the isolator switch. Place the isolator switch in the vehicle's engine compartment. The inside of the radiator support is a good mounting location. Place a screwdriver bit in the drill motor. Hold the isolator in place while you run a self-tapping screw through one side of the mounting bracket and into the radiator support. Repeat this operation for the other side of the bracket.

  3. 3

    Attach the positive (+) wiring. Run a 1-gauge wire from the positive (+) post of the new battery ("A") to the isolator switch post marked A (auxiliary). Tighten the bolt with a wrench. Place the terminal ends of the new short positive wire from the original battery "M" to the isolator's M (main) post. Tighten these connections with a wrench.

  4. 4

    Ground the system. Run a piece of 1-gauge wire from the negative post (-) of the auxiliary battery "A" in the boot to the floor of the boot. Strip both ends of the wire with wire strippers. Connect the terminal ends onto the stripped wire with the wire crimpers. Connect one end of the new ground wire to the negative (-) post of battery "A." Tighten this terminal with a wrench. Insert the screwdriver bit into the drill motor. Attach the other end of the new ground wire to the boot floor with a self-tapping screw. Run the screw through the terminal end and into the boot floor. Fasten the isolator switch's ground wire to the vehicle's chassis with a self-tapping screw. Reconnect the vehicle's original ground strap to the original battery ("M") and tighten the cable with a wrench.

  5. 5

    Power the isolator switch. Run a piece of 18-gauge wire from the IGN (ignition) post of the isolator switch to the IGN (ignition) terminal of the vehicle's fuse box. This can also be attached to any fuse that is energised only when the key is turned to the on position. Strip both ends of the wire with the wire stripper. Use crimping pliers to attach the terminal ends to both ends of the wire. Connect one end of the wire to the isolator IGN post. Tighten it with a wrench. Push the other end of the wire to the fuse box. Close the bonnet and boot.

Tips and warnings

  • When taking a long trip, turn the isolator switch to the "Charge both batteries" position. This will ensure that both batteries are fully charged.
  • Never use household wire on automotive wiring projects. Doing so could short out and harm the components.

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