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Best way to wire my boat with dual batteries

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  • Best way to wire my boat with dual batteries

    Hi there,

    I have a 17ft Microplus with two batteries and a 50hp Merc from 1988. It is currently wired with the second battery as a standalone to power horn, lights and some 12v cigarette lighter points. The second battery hooks to the engine straight.

    I'm switching it to get the second battery on to the charging circuit using a Vetus 350A switch and some 35mm2 (AWG2) wire. I know how I'm going to wire the batteries up no worries. I'll use the Vetus Switch to crank from both batteries or the main battery, charge on both, then switch to number 2 battery when moored up for the radio, fishfinders etc....

    I'm just a bit confused about wiring up my bus system behind the panel to feed 12v to all the ancilliaries. Should I hook everything to the bus then the hot wire in the ignition loom so I need to turn the key to power all the lights, horn, radio etc..... or just hook them straight into the 12v feed from battery 2?

    I'm guessing it would be safer (ie less likely to run the battery down by accident) to hook all the bits to the 12v hot wire in the tacho plug? Is that a good idea? On my engine is this the yellow wire in the tacho plug? I think the ground must be the black on that loom. By my reckoning I can just take the ground and the 12v hot from the tacho loom to my bus bars and hook all the various ancilliaries (radio, lights etc...) to that? I have a Faria tach to hook in which I could then hook to the bus bars then just run the pulse line from the tacho plug loo thing? Will this all then just work off the ignition key going to position 1? If so it would be just like a car and a good system, is this the correct way to go about it?

    Any ideas would be gratefuly recieved.
    Cheers
    Al


  • #2
    Re: Best way to wire my boat with dual batteries

    LOL Not sure what a lot of the stuff you are talking about, but you need to wire the lights and horn direct to power. Not through the switch, so if you are anchored and need the navigation lights or your horn. I have my radio through a switch, not the ignition, for the same reason. Hope that helps.
    sigpic "Spend a little money, save a lot of time"

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    • #3
      Re: Best way to wire my boat with dual batteries

      ALL accessories (meaning everything currently connected to the +12 volt buss system) get connected to the COM terminal on the switch. This allows the most versatility for the entire system. You can start the engine on BAT 1, BAT 2, or BOTH if you like. You can operate accessories from BAT 1, BAt 2 or BOTH if you like, and you can charge BAT 1, BAT 2 or BOTH if you like depending on what switch setting you use. You NEVER power accessories other than the console instruments from the ignition switch. That switch is not capable of handling high current loads. Besides, the source of power for that circuit would be from whatever battery you have selected, then to the engine, then up the engine harness, through the ignition switch to the accessories on too small a wire. Here is how you do it. The only thing not shown is the heavy gauge wire (#8 or 10 gauge) from the COM terminal to the fuse or cirucit breaker panel. Nothing other than a trolling motor or an automatic bilge pump should be connected directly to either of the batteries.

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      • #4
        Re: Best way to wire my boat with dual batteries

        Thanks for your reply, now you mention it if i will end up with loads of current going through that thin wire, thats not good! Ill check my switch but nit aware it has a COM terninal? Or am i being thick? Mine has three terminals, is the com one of those or a position for thd switch? Excuse my ignorance !
        Cheers
        al

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        • #5
          Re: Best way to wire my boat with dual batteries

          sorry I am being thick, I understand now. Apologies for double post. I'll wire the bus to COM using a decent gauge wire. I guess COM means Common? Can I just hook my earth/ground bus bar to the 2nd battery -ve clamp? If so what gauge wire should I use?
          Cheers
          Al

          Comment



          • #6
            Re: Best way to wire my boat with dual batteries

            The negative terminals on both batteries must be connected together and then to the engine block. Those ground connections need to be the same size or larger cable as the large battery cables that go to the engine. Same for the cables between the positive terminals of the batteries and the switch. The groud/earth bus bar needs to be connected to EITHER of the negative battery terminals. Wire size should be minimum #10 gauge but #8 would be better. Even #6 would be ok as bigger is always better. Wire size should always be calculated based on the maximum current the circuit will carry. Add up the current draw of all your accessories. Chose a wire size that can safely handle that amount of current. You should also have a circuit breaker or fuse in the line from the COM terminal on the switch and the POSITIVE buss. Again, that wire and the breaker/fuse must be sized for the TOTAL electrical load of the accessories. Each accessory is then fused off the positive buss for that accessory only.

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            • #7
              Re: Best way to wire my boat with dual batteries

              Thanks very much for your help

              Regards
              Al

              Comment



              • #8
                Re: Best way to wire my boat with dual batteries

                Its now getting common practice on dual battery installations where both are charged/discharged together to ensue that the cables feeding both batteries are the same length. So in the diagram above two negative wires from the engine one to each battery would be considered good practice and keeping the two lives equal lengths also is the trend.

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                • #9
                  Re: Best way to wire my boat with dual batteries

                  Originally posted by fucawi View Post
                  Its now getting common practice on dual battery installations where both are charged together to ensue that the cables feeding both batteries are the same length. So in the diagram above two negative wires from the engine one to each battery would be considered good practice and keeping the two lives equal lengths also is the trend.
                  Interested to hear your reasons for this. Not necessary at all to have equal length cables. If you are woried that the extra voltage drop in the longer cable is somehow going to affect the battery getting completely charged, that isn't going to happen. Once the charge current tapers off, the voltage drop difference will become negligible.
                  1998 Wellcraft Eclipse 24 Cuddy
                  Volvo Penta Duo-Prop 7.4L "LK"

                  2006 Sun Tracker Party Barge 21
                  Mercury 90 4-Stroke FI
                  "Common sense is not very common"
                  "Failing to prepare is preparing to fail." -- John Wooden

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                  • #10
                    Re: Best way to wire my boat with dual batteries

                    yes I understand what you say as I was not born yesterday but there is discharge as well as time to charge to consider....balance is the current term ..(as in modern) ..If you want to stick with old school thought thats fine with me

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                    • #11
                      Re: Best way to wire my boat with dual batteries

                      Originally posted by fucawi View Post
                      If you want to stick with old school thought thats fine with me
                      How can charging or discharge matter with a hundreth of ohms resistance difference? Each batteries internal resistance will vary more than what the extra resistance of a few feet of cable is going to introduce. Are you somehow worried that one battery is going to get charged with a few more milli amps of current than another?

                      Maybe explain your theory in engineering terms. Would love to hear "Where the" Fucawi is coming from!
                      1998 Wellcraft Eclipse 24 Cuddy
                      Volvo Penta Duo-Prop 7.4L "LK"

                      2006 Sun Tracker Party Barge 21
                      Mercury 90 4-Stroke FI
                      "Common sense is not very common"
                      "Failing to prepare is preparing to fail." -- John Wooden

                      Comment



                      • #12
                        Re: Best way to wire my boat with dual batteries

                        interesting you talk of hundredths of ohms in the wires ..you illustrate the point exactly ..the internal resistance of the battery is a few thousanths of an ohm .....hence in discharge the current drawn from each battery is different ( when in parallel) if the cables are different resistances and therefore the charge requirements are different making the batteries go out of balance....
                        you know the saying ....non so blind as those who cannot see ......or cannot see the wood for the trees...
                        Attached Files

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                        • #13
                          Re: Best way to wire my boat with dual batteries

                          Originally posted by fucawi View Post
                          interesting you talk of hundredths of ohms in the wires ..you illustrate the point exactly ..the internal resistance of the battery is a few thousanths of an ohm .....hence in discharge the current drawn from each battery is different ( when in parallel) if the cables are different resistances and therefore the charge requirements are different making the batteries go out of balance....
                          you know the saying ....non so blind as those who cannot see ......or cannot see the wood for the trees...
                          Agree with the diagram and the discussion. But it is "right" to the point of insignificance and no consequence in the recreational boating application.

                          Comment



                          • #14
                            Re: Best way to wire my boat with dual batteries

                            I could find you a few thousand narrow boat owners with dual battery set ups who would not agree with you as well as larger installations on boats with fridge.TV etc etc.

                            Comment



                            • #15
                              Re: Best way to wire my boat with dual batteries

                              Fucawi(Prince of Darkness)...Curious what your background is. I know you worked for Lucas from what you stated in an earlier thread. Were you a technician?

                              In every one of your pictures there is a battery that will have a different voltage drop because each battery sees an effective different cable length. Maybe your idea of a "matched" cable length differs from mine.

                              By the way, the internal resistance of a fully charged lead acid battery is around 0.01 ohm and changes from battery to battery and the battery's state of charge so good luck on matching the impedance which is what you are claiming you are trying to do.

                              Your idea of a battery going out of balance is a curious statement as well. A 2 AWG cable has only .0001563 ohms/foot. You would have to have a huge difference in battery cable lengto to be at all significant.
                              1998 Wellcraft Eclipse 24 Cuddy
                              Volvo Penta Duo-Prop 7.4L "LK"

                              2006 Sun Tracker Party Barge 21
                              Mercury 90 4-Stroke FI
                              "Common sense is not very common"
                              "Failing to prepare is preparing to fail." -- John Wooden

                              Comment

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