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Battery Isolator

kordokordo Member Posts: 229 ✭✭
in General Boating Discussions
Any electrical experts out there?  Conventional wisdom has it that you should not turn off a battery if the engines are running because you will damage your alternator.  What happens in the following situation?  I have a port alternator and a starboard alternator connected to a battery isolator.  On the output side of the isolator, I have a port battery, a starboard battery and 2 house batteries.  What happens if the output wire to the house batteries comes off while the engines are running?  I think this is the same as turning off the battery switch.  Will this harm the alternators?  I don't think so since the isolator protects the current from going back into the other batteries so, the alternators would not know that the house batteries were disconnected.  Am I missing something?  Thanks.
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    GrahamuGrahamu Member Posts: 865 ✭✭✭
    edited November 2022
    It will not harm your alternators but the wire that becomes disconnected will be live and can cause a fire if it touches a ground point.
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    Dream_InnDream_Inn Member, Moderator Posts: 7,559 mod
    I agree with Grahamu, it will not harm the alternators, but it is something you need to caution with the loose wires.

    Dream 'Inn III -- 2008 400 Express

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    Liberty44140Liberty44140 Member Posts: 4,329 ✭✭✭✭✭
    Yep I leave my battery switches on all summer and I have the battery charger on while running my engines and genny. No harm or issues 
    07' Cruisers 390 (Previous Rinker's: 06' 342EC & 01' 310FV)

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    kordokordo Member Posts: 229 ✭✭
    Ok, if my alternators won't be harmed, then is there a problem with using 2 110ah lifep04 batteries as my house bank.  Other than the charging profile issue, if the bms (battery mgmt system on lifep04 batteries) disconnects them, my alternators should be fine.  Just fiddling with some low-cost ways of adding more battery capacity.  Thanks.
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    GrahamuGrahamu Member Posts: 865 ✭✭✭
    Your alternators will never know, the sense wire for them is connected to your engine batteries and their state determines what the alternators will put out. Assuming nobody has messed with the wiring.
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    kordokordo Member Posts: 229 ✭✭
    Grahamu, do you happen to have a pic of the sense wires you referred to?  My boat is put away for the winter so I can't check for myself.  Thanks.
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    GrahamuGrahamu Member Posts: 865 ✭✭✭
    No I don't, But I doubt anyone would have changed the engine harness. I will look in the engine manual to see if there are any diagrams or pics.
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    GrahamuGrahamu Member Posts: 865 ✭✭✭
    It's the red/purple and purple wires.


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    kordokordo Member Posts: 229 ✭✭
    Thanks.  Looking at the isolator side of the drawing, there must be another wire going to the cranking battery at location 1.  I'll take a good look at my boat in the Spring or maybe the winter to better understand my setup.  Thanks again.
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    YYZRCYYZRC Member Posts: 4,908 ✭✭✭✭✭
    edited November 2022

    Edit: @Grahamu is correct there was an additional connector. 
    Post edited by YYZRC on
    2008 350 EC on Georgian Bay
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    GrahamuGrahamu Member Posts: 865 ✭✭✭
    @YYZRC, you should also have a sense wire to control the output of the alternator
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    YYZRCYYZRC Member Posts: 4,908 ✭✭✭✭✭
    edited November 2022


    The connector that @Grahamu is referring to is top center.
    2008 350 EC on Georgian Bay
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    GrahamuGrahamu Member Posts: 865 ✭✭✭
    @kordo,kordo said:
    Thanks.  Looking at the isolator side of the drawing, there must be another wire going to the cranking battery at location 1.  I'll take a good look at my boat in the Spring or maybe the winter to better understand my setup.  Thanks again.
    The wire from B1 (Isolator) to the starter relay (d) is the connection to the cranking battery as shown on the lower left side of the drawings.
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    kordokordo Member Posts: 229 ✭✭
    I think we're saying the same thing although I don't get your comment regarding the lower left side of the drawings.  There is only one drawing attached.  Only page 4c-14 came through in the post.  Thanks.
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    GrahamuGrahamu Member Posts: 865 ✭✭✭
    edited November 2022
    There are 2 circuit diagrams within the drawing, I was referring to the left schematic as how the isolator is connected to your cranking battery 
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    kordokordo Member Posts: 229 ✭✭
    Ok and not to belabor the point, but there is no isolator in the left schematic.  In any case I understand what you are saying.  Thanks.
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