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    Alternator output goes to battery switch

    I just replaced my stbd alternator with a 105 amp unit. The existing wiring is #6, so I am changing this to #2. The boat came equipped with an isolator which sends the alternator output to the starting battery (via the starter) and the house battery via the main battery switch. I am wondering why it would be necessary to go to the MBS. There should be no voltage at the switch unless the engine is running (alternator putting out voltage). I can't see any circumstance where I would turn off the MBS with the engine running. I am thinking of just running the wire directly to the house battery. Any thoughts?
    Kevin
    Seattle
    'Joysea' 1998 3788 MY 270 Cummins
    My 6th Bayliner

    #2
    My 3587 is wired the same way. I never understood it but learned to accept it!

    Comment


      #3
      Unless the field in the alt is also switched, the last thing you want to do is open the output of the alt. That is what destroys diodes.

      Comment


        #4
        The isolator that you mention, is it the old style with cooling fins and 3 terminals?... if so, it is not good for the batteries, as you can never get the batts fully topped up with that type of isolator... as to the alternator wire, connecting it to the batt switch is acceptable...
        the batteries shouldnt have components connected directly to them to keep them as clean and organized as possible...

        An ACR unit would be the proper type of isolator to use, as it can help keep both banks fully charged whenever there is an active charge source...


        NU LIBERTE'
        Salem, OR

        1989 Bayliner 2556 Convertible
        5.7 OMC Cobra - 15.5x11 prop
        N2K equipped throughout..
        2014 Ram 3500 crew cab, 6.7 Cummins
        2007 M-3705 SLC weekend warrior, 5th wheel
        '04 Polaris Sportsman 700 -- '05 Polaris Sportsman 500 HO
        Heavy Equipment Repair and Specialty Welding

        Comment


          #5
          It’s my understanding that it’s been a standard practice in the industry for the port engine alternator to go to the starter solenoid to change the starting batteries. The starboard alternator then must bypass the solenoid and go to the MBSS to separate the charging to the house batteries. You want to be able to switch the alternator off at the MBSS, or in case of a problem with the port alternator, you can still charge the starting batteries.
          P/C Pete
          Edmonds Yacht Club (Commodore 1993)
          1988 3818 "GLAUBEN”
          Hino EH700 175 Onan MDKD Genset
          MMSI 367770440

          Comment


            #6
            So in my case, the port alternator goes to the port batteries and the starboard alternator goes to the battery switch. So backwards!

            Comment


              #7
              Each of my start batteries are fed directly from the starter wire, which is fed from its respective isolator.
              I know I should use an ACR but am dreading hooking up the starter cutout wiring, which has to go to the ignition key or starter solenoid. Since this is the stbd side I don't want to spend any more time stuck in there. Also, why does it require a starter cutout since the alternator doesn't put out voltage until the oil pressure comes up?
              Kevin
              Seattle
              'Joysea' 1998 3788 MY 270 Cummins
              My 6th Bayliner

              Comment


                #8
                Why would you prefer an ACR over an isolator? Isolators seem better: alternator output will automatically go to both batteries (ACR does this, too), the house won't drain down the start battery (ACR does this, too), but you avoid the potential for a severely drained bank to cause an excessive balancing load from one battery bank to the other when the ACR closes.
                Brad
                Boatless in Seattle
                Hunting for 4588

                Comment


                  #9
                  Think its best to route #2 battery switch to keep boat as built for insurance purposes, just in case. Believe old standard was to have 1 disconnect between batteries and supply circuits including alternator.
                  Forward to 2020.
                  ABYC says you need fuse within 2 feet of alternator and 2nd fuse within 2 feet of battery. This protects a short between alternator to ground and battery to ground.
                  Think if you use the 2 fuse solution you have current ABYC standards and can bypass battery switches.
                  Use #2 between alternator and negative battery leads on engine block.

                  The diode type isolator has forward voltage drop approximately 0.5 to 1.0 volt. If alternator is internally regulated batteries will not be charged fully because regulator is reading voltage before diode.
                  If externally regulated and sense wire is connected to start battery side this corrects the low voltage on start battery. However the house battery could be under charged.
                  Running 2 alternators the diode isolator is not required. If you want to run both alternators in tandem, a charging relay (acr) is the way to go. Or you can cheat and place battery interconnecting switch on, remember to turn it off when engines shut down, to separate house and start batteries.
                  If your going to install 105 amp alternator go with external regulator. Price should be close, Single stage truck external regulators are inexpensive. Most external regulators have voltage adjustment. Plus later if you want to go with 3 stage regulator its a simple wiring change.
                  Rod
                  1991 4387
                  Sidney, BC

                  Comment


                    #10
                    Thanks for the input, I decided to switch both sides out to use an ACR since I recently switched start batteries to group 31 AGMs and I don't want them undercharged.
                    Kevin
                    Seattle
                    'Joysea' 1998 3788 MY 270 Cummins
                    My 6th Bayliner

                    Comment


                    • PFW
                      PFW commented
                      Editing a comment
                      When you use an ACR make sure all batteries that are combined are of the same chemistry. Different battery chemistries have different charging profiles...

                    #11
                    Originally posted by purduepilot View Post
                    Why would you prefer an ACR over an isolator? Isolators seem better: alternator output will automatically go to both batteries (ACR does this, too), the house won't drain down the start battery (ACR does this, too), but you avoid the potential for a severely drained bank to cause an excessive balancing load from one battery bank to the other when the ACR closes.
                    I can tell you the answer or you can google the difference between an acr and an isolator... either way, the acr is the way to go.. with the new and better technology, it makes the old style battery isolators good for nothing but scrap iron...


                    NU LIBERTE'
                    Salem, OR

                    1989 Bayliner 2556 Convertible
                    5.7 OMC Cobra - 15.5x11 prop
                    N2K equipped throughout..
                    2014 Ram 3500 crew cab, 6.7 Cummins
                    2007 M-3705 SLC weekend warrior, 5th wheel
                    '04 Polaris Sportsman 700 -- '05 Polaris Sportsman 500 HO
                    Heavy Equipment Repair and Specialty Welding

                    Comment


                      #12
                      Now that I have the ACRs installed I am a bit concerned because it is combining the batteries when I am on shore power. Since I have lead acid house batteries and AGM start batteries I don't want them combined 24/7. Norton Rider said he talked to an AGM manufacturer and they said it wouldn't hurt the AGMs short term while the altenator was charging, but I agree it can't be good charging them 24/7.
                      Kevin
                      Seattle
                      'Joysea' 1998 3788 MY 270 Cummins
                      My 6th Bayliner

                      Comment


                        #13
                        Originally posted by Centerline2 View Post

                        I can tell you the answer or you can google the difference between an acr and an isolator... either way, the acr is the way to go.. with the new and better technology, it makes the old style battery isolators good for nothing but scrap iron...
                        Based on my research the FET isolators are way better than the ACRs, and the ACRs are way better than diodes. Maybe I was using ambiguous terminology--I did not mean the older diode type.
                        https://www.pysystems.ca/products/po...tery-isolator/
                        Brad
                        Boatless in Seattle
                        Hunting for 4588

                        Comment


                          #14
                          Think you could wire control circuit of ACR to switched side of ignition switch. That will stop ACR paralleling batteries when sensing charging voltage from inverter / charger, when engine stopped.
                          Rod
                          1991 4387
                          Sidney, BC

                          Comment


                          • uffdakev
                            uffdakev commented
                            Editing a comment
                            The Blue Sea ACR doesn't have any way to stop it from combining except when it receives power from the starter side of the ignition switch. To do what you suggest, there would have to be a way to have power to that connection until the engine starts and then disconnect power while the engine is running.

                          #15
                          For now I am going back to my isolators. I have been turning on my 'battery boiler charger' for a few hours now and then. Hopefully this will help bring up the AGMs voltage.
                          Kevin
                          Seattle
                          'Joysea' 1998 3788 MY 270 Cummins
                          My 6th Bayliner

                          Comment

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