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Dual Battery Voltage Testing

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  • Dual Battery Voltage Testing

    Hi Guys,

    First time poster, long time reader.

    I've just installed a dual battery in my 120 series using a Redarc SBI12 isolator. The battery is in the boot. I have run 6b&s cable and fused it as per Redarcs guidelines. The negative runs all the way back from the auxiliary battery to the earthing point in the engine bay.

    When the car is running and I test the voltage at the starting battery it is around 13.90. When I test the voltage at the end of the cable run in the boot it is at about 13.85 so pretty close. When I plug the auxiliary batter onto the system the voltage on the aux battery is shown at 13.5 so about a 0.4V drop from the starting battery. Why would this be happening if there is minimal drop when the aux battery isn't plugged in?

    Also, I see a lot of you install the voltage boosting diode. Do these work, is there any potential to damage the vehicle electrical system?

    What type of amperage do you get from the alternator, ie how long does your aux battery take to charge?

    Cheers for the help.

  • #2
    Without the second battery there is no load . No load means no current, If you have no current you cant get voltage drop. so what you are seeing is pretty normal.


    • #3
      ah ok, thanks. So probably worth getting the voltage boosting diode.