12V/inverter/amps

Discussion in 'Tech/Mech General' started by 1948, Jun 6, 2010.

  1. 1948

    1948 Read Only Funster

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    There are 2x110Ahr leisure batteries and a 1.5KW inverter fitted in my van Am I correct in thinking that (asuming the inverter is 100% efficient) and in order to give out maximun Wattage demanded by an appliance The input cable into the invertor will have 125AMP running through it, since Ampxvoltage = Watt.
    It then follows that the wire connecting battery to inverter must be size of my finger (does not look like it visually) to be able to carry 125Amps and if an 1.5Kw appliance is plugged in will run for approx 1hr before both batteries die. or van catches fire from overheated cable before battery power is exhausted.
    Any electrical minded person out there please comment Thanks very much.
     
  2. pappajohn

    pappajohn Funster Life Member

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    correct....

    125amps and around 50 minutes before the batteries are 50% discharged (technically dead)

    the invertor supply cable would need to be around 25mmsq.
    if over a longer distance (more than a meter or so) then a thicker cable will be needed.
     
  3. 1948

    1948 Read Only Funster

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    thanks john, how to check if correct size cable is fitted Is it measuring diameter of cable and work out sq???
    I plugged in appliance 800Watt and inverter screamed and voltage on battery showed 12.2 volt immediately and went up to 12.9volt immediately when appliance is unplugged Does it mean batteries no good as it should run at least 30min??
     
  4. pappajohn

    pappajohn Funster Life Member

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    your invertor may be 1500w PEAK output which may be for as little as one or two seconds before it overloads which is probably why it was screaming.

    i think mine is a 500w peak/350w continuous invertor.

    the battery voltage will drop dramiticaly when you switch the invertor on due to the high current drain.
    as an example, your starter battery may drop to as little as 9v when you operate the starter for the same reason but stop cranking and it'll soon return to 12.6v


    dont know how to 'size' cables as we use computer software in the office to do it for us.
    shouldnt be a problem if you are up to speed on maths

    why tax your brain when a 'puter can do it in a split second !!!!

    simply ask for 25mm cable when you buy but dont buy the stuff used for 230v meter tails....its not flexible enough. you need dozens of fine strands...not half a dozen big ones.

    best place would be a car audio specialist...those large amplifiers used by kids in trick cars carry a lot of amps, or a welding supplies shop for the stuff used in arc welding equipment.
     
  5. 1948

    1948 Read Only Funster

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    thanks john
    any comment regarding the instant voltage drop of the leisure batteries when 800Watt appliance used via the invertor, should there be a time lapsed before voltage drop (say at least 10/20min)
    800Watt is well below 1.5kw inverter specs Why would it overload??
     
  6. pappajohn

    pappajohn Funster Life Member

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    the voltage drop will be dependant on the load on the invertor.

    put a 50w appliance on the invertor and the voltage drop may not even be noticable.

    put a 1500w load on and the drop will be instant and dramatic.
    remove the load after a couple of minutes and the voltage will rise again to around what it was to start with after a few seconds.

    it may be an 800w invertor with a peak output of 1500w for a second or two.

    obviously from a marketing point of view a 1500w invertor is more attractive than an 800w model so its 'labelled' as 1500w.
     
  7. 1948

    1948 Read Only Funster

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    thanks for explaining john,
    trip to shop to morrow for the correct size cable, not that it will even have 125amps running through it Better be save I guess
     
  8. vwalan

    vwalan Funster

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    does seem to me to be the batteries as well .i think to get alot from your inverter you really need twice that battery power and fully charged. i use a 2kw inverter never really at full power. it as two red two black cables like jump leads .i also have 600amp battery bank ,but its amazing how easy it is to get the inverter to squeal when the batteries are down a little bit .i use a 1000wat karcher vacum to give the trailer good clean up after being away .get it going and you can almost see the batteries voltage falling . .by the time i had finished the 100wt fridge made the inverter squeal. but you could still run the vacum. seems to be also the initial draw. sounds daft but if you have something like a tooth brush on charge allhe time the initial draw doesnt do it then. there is a name for it . my mate showed how it works in his van. i use normal tooth brush .
     
  9. olley

    olley Funster

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  10. Fatalhud

    Fatalhud Read Only Funster

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    If you don't need a 1500 watt inverter then ditch it for a size better suited to your needs
    My van came with a 1600 watt unit which is a pain because it pulls 3.5 amp when switched on and no load connected
    This drastically reduces the battery life when not on hook up for a few days
    It is now gathering dust, while I now run a 300watt unit to power what I need

    Alan H
     
  11. pappajohn

    pappajohn Funster Life Member

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    on olley's suggested website select item ref CM25 from battery and starter cables.

    that 322 strands at 0.30mm thick...nice and flexible. :thumb:
     
  12. 1948

    1948 Read Only Funster

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    I only switch it on when I need to use it and so far used for charging computer and phone, only realised to day that it can be a fire hazard I was trying to use it for a 800Watt toaster. Obvious battery?? Inverter ?? not up to it
     
  13. vwalan

    vwalan Funster

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    hi alan h you sure? my ring inverter reckons 1amp on stand by ,its 2kw. could be right but i know all the figures i have seen fall into 100wt apliance uses 10amp through an inverter. the inverter should come with the right size cable dont you think.
     
  14. Fatalhud

    Fatalhud Read Only Funster

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    That's the reading I got when I was checking what circuits where pulling what current.
    My meter is not calibrated, but I don't think its that far out

    As for checking the cable size, most modern cables have the size printed or embossed down there length

    If you are going to change the tails then ask for Tri-rated cable, which is nice and flexible

    Alan H
     
  15. BobProperty

    BobProperty Read Only Funster

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    Is this any use?
    http://www.the12volt.com/wiring/recwirsz.asp

    The car audio guys quite often use that as a reference. I'd go for the welding cable, I think you can get it off ebay quite cheaply for what it is. Warning, it's thicker than your typical battery cable so don't go thinking you can just slip it in somewhere, it needs planning for.
     
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