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battery isolator

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  • battery isolator

    I would like to install an isolator for my starting and trolling batteries. My engine is a 40hp 2 stroke cycle Mercury. First, what amp isolator do I need? Second, there are two wires connected to the positive post on the starting battery, is the small one the charge wire or do I need to come straight from the alternator?

  • #2
    The small wire is an accessory feed for the non-motor electrical gear - Radios, light, sonar, etc. Battery charging is through the big red cable.

    When you say isolator, do you mean battery switch or an actual battery isolator?
    Please, no PM's (Private Messages) regarding boat/engine problems.
    That is what the forums are for.
    Only forum/moderator issues will be answered in PM's.


    • #3
      We will need the year of the Merc as well. Some of the older ones only charged 9A at most. A battery isolator has a built in voltage drop, which will cut that charging output significantly. You therefore may not be able to effectively charge your deep cycle battery in any practical fashion.


      • #4
        An actual battery isolator. The engine is 2003.



        • #5
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          A small riding mower starter solenoid would work well wired so when the ignition switch is turned on the solenoid is energized. Simply power all accessories from the extra battery and install the solenoid in the charge wire from the engine to that battery. In the boat installation you'd need a ground wire from the engine battery to the solenoid frame, or in the case of some, to the ground terminal on the solenoid.
          Ol' Stonebreaker