Blown fuse on battery linkage

Discussion in 'Motorhome Chat' started by Sunty, Jul 28, 2012.

  1. Sunty

    Sunty Read Only Funster

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    Wondor if anyone might have a suggestion as to why the 16amp line fuse on the cable connecting my two 12 volt leisure batteries should blow when I use the inverter close to its limit, ie 700 of the 800wt possible? Happened twice now but everything works at normal mid-range usage when I replace to fuse. Inverter is fairly new and the batteries show 13.4 amp or better.
    Sunty.
     
  2. hilldweller

    hilldweller Funster Life Member

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    800 / 12 = 66A.

    Ideally you want 33A from each battery but they aren't that exact in their workings.

    A fuse connect two batteries should be more like 50A maybe more but needs cable size to suit and maybe 2 fuses depending on layout.

    What I am NOT saying is just replace the 16A fuse with a 50A one.
     
  3. Jaws

    Jaws Funster Life Member

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    Hi Sunty, batteries will be showing voltage not amperage..
    A Battery can delivery MASSIVE amperage ( up to several hundred amps for a brief time in fact ).

    Couple of things then

    An 800w inverter will draw up to 70 amps when flat out.

    Chances are very high the batteries will not be perfectly balanced, with one delivering more current than the other.
    Once that one starts to fall the current draw on the other will increase to balance the system out..
    As soon as that happens the amperage drawn can be VERY substantial indeed.

    I would say you need to put a much much larger fuse in the line ( I do not have ANY sort of fuse between batteries and am really unsure why it is actually there on yours.
    Fusing downstream of the batteries yes, but between them ?
    The only reason I can think of is the designer was VERY old school and dates back to the times when batteries sometimes went short circuit.. But the way modern leasure batteries are made makes that virtually impossible ( not completely impossible.. I learnt many years ago that a 1 in a million chance happens 9 out of ten times LOL !!)
     
  4. Jaws

    Jaws Funster Life Member

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    We were posting at the same time Brian..

    My only issue is that 66 amps.
    That would be in a perfect world with superconductor circuitry..
    reality is it is likely to be at least 5% inefficient if it is a real expensive unit, but could be as much as 25% inefficient for the cheaper ones
     
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  5. Sunty

    Sunty Read Only Funster

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    That sounds very heavy, but I understand your maths. The connecting cable was off the shelf from Todds Motorhomes and the actual fuse is not marked as to its size. However, when it first blew I simply replaced it with one of the same colour assuming this would be the same power. I have only seen 25amp fuses of the type fitted to mine, so it may mean a different cable or line fuse. What do you think.
    Manay thanks.
     
  6. Jaws

    Jaws Funster Life Member

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    In that case the first question I would ask is exactly how thick that connecting cable is ?
    To give you an idea, I have used cable about 2/3rds as thick as a cheap jump cable

    Remember, any losses in the cable will actually increase the current draw as the voltage drops !!
     
  7. Sunty

    Sunty Read Only Funster

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    Brilliant - thanks ever so much for the replies. I will check the cable by using the inverter by-passing the fuse. I think I know someone who will be able to check the power usage if I have further problems.
    Sunty. :thumb::thumb::thumb:
     
  8. Reallyretired

    Reallyretired Funster

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    You seem to be sorted now. Bear in mind inverters need big fuses as well as big wires. Mine is fused at 300A (1.5Kw continous 3Kw peak so a bit bigger than yours)
     
  9. Jaws

    Jaws Funster Life Member

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    But not everyone has a 20kVA generator on tow !! :thumb:
     
  10. Wildman

    Wildman Read Only Funster

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    Zadi rectangular twist and push locks.

    Does anyone have an old Zadi lock, knackered barrel fine I have a good one but need some other parts (the parts that actually lock)
     
  11. Jaws

    Jaws Funster Life Member

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    Well thats a bit of a leap !!

    You have a door handle and lock on your batteries or using one as a VERY high amperage link ???
     
  12. pappajohn

    pappajohn Funster Life Member

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    I've explained the reason many times john....

    if there is absolutely no possibility of the interlinking wire becoming chaffed, rubbed or cut and shorting to earth, IE: batteries next to each other, there is no need for a fuse in the pos wire.

    if on the other hand, the two (or more) batteries are seperated....IE: one under the passenger seat and one under the dinette....then there is ALWAYS a possibility of a short to earth.

    one fuse in this situation isnt enough, it needs one fuse at BOTH ends of the link wire.

    if it does short, the fuse protected battery will become isolated when the fuse pops but the other end of the link wire is still connected to a 'live' battery with no protection....likely result, fire or an exploding battery due to the massive current draw.
     
  13. Jaws

    Jaws Funster Life Member

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    Yup, gotta agree 100% with that..
    I tend to think folk do as I do ( if you know what I mean ) and when dealing with battery level amps I REALLY make sure there is no chance of any sort of chaffing or shorting.. ( almost to the level of paranoia !! )
     
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