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inverter question

Discussion in 'Electrical Engineering' started by Jim, May 10, 2006.

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  1. Jim

    Jim Guest

    Hello,
    I hope I'm addressing the correct newsgroup.
    I would like to use an electric chainsaw from my pickup truck via an
    inverter. ( I don't want to use a generator)
    The chainsaw I selected draws 1,400 watts @ 120 volts.
    The inverter is made by Vector (from the Northern tool catalog) and is rated
    for 1,500 watts output / 3,000 watts surge.
    My truck battery is 750 amp rated and is 6 years old.
    My alternator is rated at 136 amps.

    My questions are;
    Can this be done with the battery and charging system I have?

    Is Vector a reliable brand? Anything better?

    Do I have to run the engine RPM's faster than idle and by how much?

    Thanks,
    Jim
     
  2. TimPerry

    TimPerry Guest

    assuming zero loss 1400W / 13.8 V = 101 AMPs. actual load will be more.

    just my opinion now but i think you may headed down the wrong path. this is
    based on my owning an inverter with about the same ratings.

    i use 2 parallel deep cycle batteries which in turn bridge the charging
    system.
    the 2 batteries alone can run a couple sound systems for a 2 hours before
    the voltage drops below the shut off threshold. i think that running the
    chainsaw it would only last a few minutes if the saw would start at all.

    assuming you are going to the trouble is rigging this up so you can cut down
    trees for 8 hours at a time, i suspect you will find that at idle the
    alternator can not keep up with the demand and you will end up quickly
    discharging the battery. i also question the ability of the alternator to
    withstand the close to 100% duty cycle that this load would need.
     
  3. To further this thought, what you describe is an automotive STARTING
    battery, the 750A is a Cold Cranking Amp (CCA) rating, meaning it will
    put out that much current for only 30 seconds, and is NOT designed to
    be drained all the way down lest it become damaged and unable to fully
    recharge ever again. What you need to look for is an Amp-Hour (AH)
    rating, which you most likely will not find on that battery. In the
    old days, that is how batteries were sold, and from experience I can
    tell you that battery is probably good for about 60AH max. That also
    means that at 60AH the voltage will begin to drop rapidly from 11.5V,
    which in the case of your chain saw, would INCREASE the current draw
    by the motor, which will INCREASE the voltage drop rate etc. etc.
    until the saw gets hot AND stops.

    So if you look at Tim's calculation of 101Amps from your 60AH battery,
    you can run that saw for 60/101= .59 hours or roughly 36 minutes,
    before the battery would fail for you. The alternator would do little
    to stem that tide, and Tim was correct in expressing concern for
    continuous use. Most alternators can supply around 25A continuously,

    You could spend a bunch of money for deep cycle batteries and a hefty
    generator like those found on tow trucks, but in the long run it's
    cheaper to just buy a gas powered saw.
     
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