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Backup battery relay and wiring

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  • Backup battery relay and wiring

    Picked up a cheap ebay backup battery for the U3, plan being so it can run from that when the car is off so it doesn't drain the main battery. Now unfortunately it'll only take 1a charge whilst giving 2a out. So if I was to just use it, in the end it wouldn't last too long.

    What I'm thinking is to wire it up, so when the ignition is on, power it taken from the main battery, and the backup is charged. When it is off it'll switch to the backup battery.

    What would be the best way to wire this up?

  • #2
    Connect the relay coil to something that is on when the engine is running.

    A wiring diagram for your vehicle will be your best friend to figure that out.

    I know in most vehicles the fan circuit is energized only with the ignition switch in the on position, so that might be a circuit worth tapping into.

    Then use the relay contacts to connect the main and auxiliary batteries together when the coil is energized.

    You will also want a fuse in the power conductors at both the main and the auxiliary battery. Without said fuse, you might find the wire that carries the charging current between the two batteries accidentally carrying hundreds of amps if something does not go as planned during an engine start condition.

    If you want a really painless installation (for a lot more money of course)...

    check out these... I am using both in my new pickup and so far they have worked well.



    • #3
      Thanks for the suggestions. I'm learning this as a go along, still a bit confused.

      Would the above setup not still run the odroid from the backup battery when the ignition is on?

      Don't really want to spend any money on the later options.

      Do I need a 4 or 5 pin relay? I started trying to number a 5pin and now it wont let me delete the picture.
      Attached Files
      Last edited by chug1; 08-07-2014, 06:22 AM.


      • #4
        -connect your droid to the backup battery. Use a fuse right at the battery for safety.
        -Connect the two batteries together with the normally open contacts (30 + 87).
        -Connect one side of the coil to ground (85) and connect the other side (86) to something that gets turned on only when the ignition switch is in the "ON" position.

        On every car that I have seen, the power to the heater fan is only applied when the ignition switch is in the "ON" position, so that would be a good choice. Usually it runs to the fan motor thru a relay, and then the speeds are selected on the ground side thru a switch and a resistor bank. If that is how your car works, then the power going to your fan motor would be perfect choice to power the relay coil.

        You don't want something that also is powered in the "ACC" position or "Engine Start" position.

        Put an inline fuse at both battery connections for the sake of safety.

        Last edited by Rickk; 08-07-2014, 06:25 AM.


        • #5
          Cheers, but would that still not be running the odroid from the backup battery when the engine is running and charging it?

          "You don't want something that also is powered in the "ACC" position."

          Would you mind explaining why?


          • #6
            double post, apologies.


            • #7
              If the coil is energized in the ACC position, you will be drawing not only from the auxiliary battery with the engine off, but also from the main battery, which you said in your first post you don't want to do.


              • #8
                I thought it would only draw from that when the ignition is on? Like when you want to listen to your stereo when you're sitting in the car. Fair enough if it doesn't.

                Does that config you suggested charge the battery while it's using it?



                • #9
                  Yes both batteries will charge when the ignition is on and motor running, as well as power your odroid. Just remember that although both batteries and the odroid are in parallel, the alternator is also in parallel, providing the charging and usage current than is being consumed by the car and odroid.

                  This all assumes your alternator provides enough current for all car and odroid usage, if not you will be slightly discharging both batteries at the same time.

                  When you kill the ignition, the main battery positive is isolated, and your odroid will draw from your secondary battery.

                  Mine looks like this:

                  The 5 pin relay is connected to ignition, and switches a 150A or 200A relay to isolate the batteries positive leads.
                  Last edited by rray; 08-07-2014, 12:24 PM.