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  1. #1

    Default I have a goofy but real question

    I'm installing a Perko 2-battery switch on my fiberglass boat. I watched a couple of videos on YouTube and the tech kept referring to a "common ground". The video showed him connecting the black (ground) battery cables to a connection that was not the negative post on either of the battery posts but rather, this "common ground" connection. Everything else made sense, but now I'm thinking that I connected my Perko switch up incorrectly. If I need to install on of these "common ground" posts somewhere, how do I do it?
    Any help here would be appreciated !

  2. #2
    Join Date
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    Default Re: I have a goofy but real question

    Ayuh,.... There's No grounds anywhere near that battery switch, only the positive cables,.....

    The boat's Grounds should all be common,......
    Each battery to the motor, which makes the motor's block the common Ground,.....
    If more than 1 motor, there should be Ground straps from block to block,......
    Any Grease is Better,... Than No grease at All....

  3. #3
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    Default Re: I have a goofy but real question

    A lot of smaller boats don't have a separate 'common ground' and everything is taken back to the -ve battery posts. All the battery -ve's will be tied together and you just need to connect to that somehow. On larger boats and some smaller boats there will either be a -ve buzz bar where all the -ves join or a -ve 'post' that they all connect to. When you don't have a lot of wiring though it isn't unusual that everything goes back to the battery -ve posts and then you have a small -ve buzz bar behind the helm to connect the electronics to. That small bar will be wired back to the battery -ves as well.

  4. #4
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    Jersey Shore
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    Default Re: I have a goofy but real question

    Just an add on.....

    Common ground actually means "a common return path for electric current" IE the negative post on your battery.

    "one ground no matter how many battery banks (positives) you have"
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  5. #5
    Join Date
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    Default Re: I have a goofy but real question

    Well, it can get confusing. On some diagrams, liked the attached, there is a positive wire from the battery switch to the engine starter that is labeled "COM"

    Click image for larger version. 

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    CHawk 25 DLX

  6. #6

    Default Re: I have a goofy but real question

    Thanks to everyone that helped with a reply. Especially CHawk, that simplified diagram is a real help and I'll run the cables as shown. I'd like to say how much I've enjoyed this forum and glad I joined!

  7. #7
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    Default Re: I have a goofy but real question

    Assuming battery #1 is the house battery and battery # 2 is your cranking battery, you will want to pull a lead off of post #1 on the switch to run to your console, Also pull a lead directly from the negative post on battery #1 and run that to the console. Connect those to the fused distribution bus on the console, and from there power all of your equipment. Setting up that way, you will know that when you turn off the battery switch, all of your equipment will be turned off. The exception is the bilge pump, which should have the float switch wired directly to the battery. Don't power anything off of your key switch except the gauges.
    CHawk 25 DLX

  8. #8
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    Default Re: I have a goofy but real question

    " COM" or common on a switch refers to switch terminal technology, where you have normally open (NO) and normally closed (NC) terminals which indicate electrical conductivity depending on switch position, where the COM (C) is the connection point that is "common" depending on switch position to both the NO and NC contact circuits. Mechanically, it is the "swinger" between NC and NO.
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  9. #9
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    Default Re: I have a goofy but real question

    I realize that this thread is several months old. Here's my take on Marine 12 vdc systems.

    Quote Originally Posted by Licn2chill View Post
    I'm installing a Perko 2-battery switch on my fiberglass boat. I watched a couple of videos on YouTube and the tech kept referring to a "common ground".
    First of all, even while we will see the term being used in product schematics, etc........ there is no "ground" in a 12 vdc system. Only Positive and Negative.
    The term "Ground" refers to "Earth Ground" as in home and industrial Alternating Current.
    What we have on our boats is a
    "system negative common" point, of which is most often the engine block.
    To prevent excessive Negative cable/wire connections at the batteries, a system neg common terminal block can be added.
    The video showed him connecting the black (ground) battery cables to a connection that was not the negative post on either of the battery posts but rather, this "common ground" connection.
    I would like to see this video.

    Everything else made sense, but now I'm thinking that I connected my Perko switch up incorrectly. If I need to install on of these "common ground" posts somewhere, how do I do it?
    Your Perko MBSS will NOT have a Negative point of connection on it...... it will have three terminals only.
    .... terminal #1 for the cranking battery Positive
    .... terminal #2 for your house battery bank Positive
    ...."common" terminal for the cable that connects to the engine's starter motor solenoid.... as per post #5.

    Each battery bank will make it's own Negative connection to the System Negative Common point.


    Any power taken from MBSS terminals #1 or #2 will be Un-Interruptible.
    In other words, when you leave the boat unattended and with the MBSS turned to OFF, these terminals will still be HOT.
    If you moor this boat, the #2 terminal would be a great place to power your main bilge pump float switch!


    Any help here would be appreciated !
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

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