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Thread: Simple Dual Battery Wiring Diagram and Parts List - DIY for $50-100

  1. #1
    Join Date
    Mar 2007
    Dover, NH

    Default Simple Dual Battery Wiring Diagram and Parts List - DIY for $50-100

    Here is a wiring diagram for a DIY dual battery isolation system. This is a simple system that will cost $50-100 depending on what route you take.

    This simple setup will provide isolation for the installation of a dual battery setup. Typically this system is used by those who want to power aftermarket accessories like light bars, a fridge, or other components and still want to start their vehicles in the morning. This system will isolate the starting (vehicle) battery and the secondary (house) battery when the vehicle is turned "Off". In this "Off" position, the accessories will take their power from the secondary battery. If the aftermarket components are left on for too long or forgotten, the battery will die, just like in any scenario where someone may leave their lights on or radio on. In this case, however, the starting battery still has full power, since it was isolated from the secondary battery. You can now start the vehicle as normal and not be stranded because you left your fridge on for a week while camping. As soon as the vehicle switches to the "On" position, the batteries will connect and the alternator will start re-charging the secondary battery.

    In the alternate wiring method shown below we have added a three position switch. This adds a little complexity but is still easy to wire up. The hardest part is running the wires from the engine bay into the cab. In this alternate method we use a three position rocker switch to allow for automatic control as described above (Position II), manual battery isolation (Position O), and manual battery connection (Position I). This allows the user a little more control over the system. The rocker switch on Position I allows for self jump starting of the vehicle in case you run out your primary battery by listening to the radio too long or leaving your headlights on. If you plan to go this route, I would highly suggest the 500amp rated solenoid and fuses. This is arguably the best benefit for a user controlled system. If you use the simple wiring method, the one without the rocker switch, you can self jump start with a pair of jumper cables from the secondary battery to the primary.

    If you wanted to make this setup even cheaper you can ditch the 200amp equipment listed below and size everything for 80amps between the batteries. You can use THIS solenoid and save about $25. You will also save some money on cables (buy a smaller gauge) and fuses. This setup will NOT be able to jump start you. Just carry a pair of jumper cables.

    Click on diagram for full-size.
    Dual Battery DIY Wiring Diagram and Schematic for setting up a dual battery system in a vehicle.

    Main components:

    • Wiring - 4ga or 2ga automotive wire.
    • Connectors - Ends to put on the wires.

    • Isolator Relay:

    PAC-200 Isolator relay
    - $43.60
    . This model is a 200amp model and should be sufficient for everything except for winching.
    PAC-500 Isolator relay- $70.62. This model is a 500amp model and should be sufficient for everything including winching.

    • Fuse/Circuit Breaker:

    200 Amp Fuse - $5.99. You will need this fuse and the following fuse holder. This is a slightly cheaper method than going with a circuit breaker. This setup is a little more reliable than a circuit breaker (no moving parts) but comes at an increased cost if you do start to blow fuses.
    Fuse Holder - $4.99.
    200 Amp 12v Circuit Breaker - $16.98. This is a 200 amp circuit breaker. I like to use circuit breakers because they can be reset easily without additional cost. There is no need to carry spare fuses and the circuit breaker can act as an additional switch if necessary.

    To Do:

    Once you have everything, it is just a matter of wiring it up! From the start, we are not going to touch any of the existing vehicle wiring. We are simply going to add on to it.

    1) The first step is to get your new battery installed where you want it. Once it is physically mounted, run a wire of proper size from the negative terminal to a chassis ground within a foot of the battery tray. You want this wire as short as possible. Int he diagram above, I show that the batteries are connected by a wire. They do not need to be as long as they are both grounded. Electrically, it is the same.

    2) From here, go ahead and find a good place to mount your fuses or circuit breakers, depending on which method you choose. You want these as physically close to the batteries as possible. The shorter the wire, the less chance of a short. Once the fuse blocks/breakers are mounted, go ahead and connect a short positive cable from the positive terminals on the battery to one side of the fuse block.

    3) From here, find a place to mount your isolator relay/solenoid. Ideally, you want this to be close to the batteries as well. Having everything mounted together has some benefits, primarily shorter wire runs which saves on initial cost and also reduces points of failure. If you have 4 feet of wiring running through holes and through components and brackets you increase the chances of wear from abrasion, nicks, etc. By keeping wire runs short you are also limiting the possibility for excessive voltage drop and allowing yourself to use a smaller gauge wire. Once the solenoid is mounted, go ahead and run the primary wires from each terminal back to their respective batteries.

    4) Go ahead and run a small ground wire from the solenoid/isolator from the ground terminal to ground. The terminals are outlined in the diagram above. The wiring diagram shows a PAC-200 solenoid. If you use a different solenoid, or even if you use a PAC-200, double check the manufacturers instructions. They could change they way they make it and terminals could move. This ground wire doesn't have to be very large gauge since it carries very little current. You are going to be fine with a 12ga wire. Again, try to keep the run short.

    5) Now, the last and trickiest part. You need to run a wire from the solenoid, which is likely in your engine compartment, real close to your battery, all the way to your fuse box. Route the wire carefully making sure it isn't sitting on anything sharp or hot. The vehicle vibrates and bounces around and if it is laying on a sharp bracket it will have the insulation worn off over time, creating a short. Bad! Pay attention to where you run it and run it inside sheathing if necessary. When you get to the fuse box, choose a circuit that is only powered when the key is in the "ON" position. You can test this with a meter or test light. Make sure the circuit/fuse has no power when the key is turned off, otherwise you just installed a very expensive, non-isolating, extra battery that will drain power overnight. This is the opposite of our goal. You can use an "add-a-fuse" block once you have found a fuse or you can wire it in however you would like. Make sure the connection is good and will stand up to the rigors of off-road driving.

    Go ahead and test the system once it is installed. With the key in the "ON" position, both batteries are connected, charging, and running your components. With the Key in the "OFF" or "Acc" position, the secondary (house) battery will be disconnected form the first.

    Once it is tested and working, go ahead and wire up all of your aftermarket components to the new, secondary battery. Leave the primary battery for starting and running critical vehicle functions only. You don't want to kill your primary battery. :)
    Last edited by Ryan; 01-09-2013 at 12:37 PM.

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