Battery Cable Size?

Discussion in 'Electrical Systems' started by Katoh, Oct 20, 2011.

  1. Katoh
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    Katoh Senior Member

    Good evening All
    A very quick question, I have a run of approx 5.5m from battery's to Motor. Motor is a Volvo Penta AD31B and I have a twin Battery system mounted in the cabin.
    What size cables do I need use from batteries to motor. These are for the main start cables.
     
  2. michael pierzga
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    michael pierzga Senior Member

  3. philSweet
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    philSweet Senior Member

    You are probably better off to buy a new battery and install it near the motor. Depends on how the switching is laid out I suppose. 5.5 meters one way, right, not the total of hot and neutral. When sizing, you need the length of each wire segment with the size of each considered individually. The neutral is usually shorter than the hot. without a schematic showing batts, fuses, switches, motor, and other loads, and the cable lenghths between each, I doubt anyone will offer you anything specific.
     
  4. Frog4
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    Frog4 Proletariat

  5. messabout
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    messabout Senior Member

    Are batteries contained in the cabin a hazard? During charge cycles there are some emissions that are not healthy
     
  6. Katoh
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    Katoh Senior Member

    Gentlemen
    Thanks for the replies, To clarify the batteries are in a cuddy cabin, a non habitable area and are also vented separately to out side not into the cabin.
    The cables run from a common buss, both positive and negative to the starter on the motor. approx run of 5.5m each cable. The Ad31B motor is a 4cylinder 2.4l Diesel and I am unsure of its output in watts, Hence my question.
     
  7. Katoh
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    Katoh Senior Member

    Gentlemen
    I finally found that may starter is 3.1Kw @ 12v, this gives a phenomenal rating of 260A and according to the calculators at 5.5 meters a cable size of 110mm2 is required, this seems mighty excessive. That is larger than 4/0 AWG that can't be correct. There must be a start load and run load, constant 260A it sounds excessive.
     
  8. michael pierzga
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    michael pierzga Senior Member

    The picture is of an MTU cable sizing guide for alternators and starter motors.. Hope you can read it.
     

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  9. Katoh
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    Katoh Senior Member

    Thanks Michael
    I download the photo and photo and enlarged it was very (readable) if such word.
    Yep even according to your charts 120mm2 is suggested. Definitely some big bloody cable!
    Funny thing is that's all 24v my 12v should even be bigger.
     
  10. michael pierzga
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    michael pierzga Senior Member

    Might be good to ask Volvo Tech service for cable size guidance. I rarely see big cables on yachts and suspect that a starter motor cable specs are for momentary load
     
  11. Frog4
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    Frog4 Proletariat

    battery cables are NOT just for starting your engine. If you have an alternator, they are for charging, as is anything else run from the battery; cabin lighting, gauges, displays etc ...
     
  12. Frog4
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  13. aranda1984
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    aranda1984 aranda1984

    Cable sizing

    Starter motors can draw 7-800 Amps or more from your battery as a momentary load.

    If you undersize the cables they will heat up a bit, but you will get away with a lot here.

    The best advice was to ask the engine rep. for the appropriate cable size!

    About charging..

    If you try to put this much current into your battery, you boil away the electrolyte. (The acid.) This means losing current capacity, as the plates are no longer submersed into the acid.

    Your alternator was designed to regulate the current and voltage output to prevent this.

    Check it out, your alternator will be somewhere around 160 or so Amps. (Give or take!)

    ... All the other current loads are negligable on a small vessel, compared to the starting current!
    Once the engine is all fired up and you are on max RPM, the alternator handles the load.
    The battery is like a momentary parallel storage device to even out the over supply or the overuse of the power necessary to run everything.

    Regards,

    Stephen I. M.
     
  14. BertKu
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    BertKu Senior Member

    If you are that far away from the batteries, I can only suggest as thick as possible, But marine cable is very expensive, thus I use the biggest size welding cable avialable in this country at +/- 14 USA dollar per meter. It can handle easy 1000 Ampere . Everybody will be up in arms now, to use bare copper cable in a marine environment. Correct, however one must in that case use an English product Starbrite electrical tape and seal all bare copper ends after the slug is soldered to the cable. You seal the whole rootiemetootie with that Starbrite. Even your connections and make sure that no part can be exposed to the salty air. It works.
    BertKu

    P.s. With bare copper cable I mean the flexible pvc welding cable colour red with 12 mm thick copper diameter
     
    Last edited: Oct 21, 2011

  15. aranda1984
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    aranda1984 aranda1984

    Using welding cable

    There is nothing wrong with copper welding cable, as far as you have the right cross section area.

    The more expensive cables will have every single wire in the multi strand bundle, zink coated to prevent corrosion.
    The individual wires might be smaller diameter (more wires per same area in the bundle) for more flex, the insulation might be softer or thicker... oil resistant compound, plus many variations that will drive the price up. (Try getting double odd aircraft cable, you will get a heart attack when you see the price!)

    Repeated overheating might accelerate the unclad copper wire corrosion, especially between the crimped cable ends and the wires. This in turn introduces a resistance point that will greatly limit the current carrying capacity.

    Take care if you solder the wire ends (wire lugs) onto the cable, the capillary action between the strands will pull the solder into the cable and reduce it's flexibility.
    As long as you seal (insulate) the cable end, so that no moisture or air can get to it, you will have a perfect connection.

    Regards,

    Stephen I. M.
     
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