Wire and fuses for linking 2 batteries

Discussion in 'Tech/Mech General' started by RichardB, Jun 26, 2016.

  1. RichardB

    RichardB Funster

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    hi. In preparation for installing a new 2nd battery I have been looking at where it can go in the van. The current battery is in an outside battery box, the 2nd one is going inside under a bench seat. Physically it is located next to the 1st battery but to link the the cables will need to be 3 ft long to get in and around to it..

    what size cable would be best to join them with and what size fuse should I put between then? The fuse from the battery to the van controller is a 25amp so I presume a 25a,p between batteries should be fine?

    Thanks
    Richard
     
  2. Boringfrog

    Boringfrog Funster

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    Don't think you need a fuse between batteries.
     
  3. autorouter

    autorouter Funster

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    In theory 2.5mm squared (2.5mm2) will pass 25amps without overheating. However you need to minimise voltage drop to ensure both batteries are fully and equally charged. I'd use 10mm2 wire for both the positive and negative.

    However 6mm2 is the biggest you can use with the popular yellow crimp terminals so there's a case for using that if you can't/don't want to use copper tube crimp terminals and heatshrink sleeving.

    Be sure to put a fuse at both ends of the positive wire. If you don't, there's a risk of fire if it gets shorted to the bodywork. 25 amp fuses would be fine for this application.
     
  4. autorouter

    autorouter Funster

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    If the batteries are right next to each other and the wires are short you don't. If there's any risk of them being damaged and shorting then a fuse at both ends is the best way.
     
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  5. Boringfrog

    Boringfrog Funster

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    My live wire runs through trunking, so hopefully no risk of touching any metal bodywork.
     
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  6. RichardB

    RichardB Funster

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    Thanks all. My wire is going through trunking so should be safe without a fuse
     
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  7. Robert Clark

    Robert Clark Funster Life Member

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    Our batteries are connected with a 25mm cable - you can buy them on eBay, made any length with connectors already attached
     
  8. tonyidle

    tonyidle Funster

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    The cable needs to be no thicker than the cable that connects your existing battery to the van electrics. Volt drop over three feet is negligible.
     
  9. Techno

    Techno Funster Life Member

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    There is a minimum size but no real maximum other than what becomes impractical.
    I prefer welding cable which effectively makes multiple batteries think they are one unit.
    Best to take the load from the pos of one and the neg of the other or last in the bank.
    This is two of my front four.
    [​IMG]
     
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  10. Boringfrog

    Boringfrog Funster

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    Very Neat.
     
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  11. Techno

    Techno Funster Life Member

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    And the other half of the front four
    [​IMG]
     
  12. Techno

    Techno Funster Life Member

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    I bet you're think front four? :LOL:
     
  13. pappajohn

    pappajohn Funster Life Member

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    You don't.....but only a fool won't.
    If the linking wire should be damaged and short to earth it will become a high current electric fire element.

    A 30a fuse AT BOTH ENDS of the link wire will be more than sufficient......unless there's an inverter connected then they need to be 100a/200a/300a fuses depending on inverter size.
     
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  14. Techno

    Techno Funster Life Member

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