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Thread: Power Inverter to a truck battery (need help and ideas)

  1. #1
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    Power Inverter to a truck battery (need help and ideas)

    Hey guys,

    I bought a Roadpro RPPI-2500W 2000/5000 Watt DC to AC Power Inverter from Ama$on ($100 by the way) and would like to run a spot cooler of of it. Spot cooler is 690 watt 6 amps. My alternator is rated at 100 amps. Efficiency of an inverter is 87%. Do you think if truck will be running for 9-10 hours a day (FedEx truck) alternator will still be able to keep my batteries at above 50% state of charge so that they don't deteriorate?

    I was calculating it like this: (690w/12v)/.87= 66.1 amp is required from my battery to run it.

    so if an alternator rated to produce 100 amps it should be fine?

    Your thoughts guys?


    Thanks,
    B.

  2. #2
    FLAC SNOtwistR's Avatar
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    Should you not include what the truck is using to actually run for those -10 hours in your calulations as that will change your numbers, especially if you are using heater or a/c ect. SNO

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    FLAC PhilG's Avatar
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    Unless there is some other reason not listed; I think you would be much further ahead to run a 12v "coolatron". They are designed for automotive use, bumps & banging around etc, plus much more efficent that converting dc to ac to operate. I'm not sure what a spot cooler is but maybe it does something a coolatron won't?
    My 2007 Ford F350 Work Log located HERE

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    Very sorry guys for not including all the info.

    Truck doesn't have an air conditioner in it and with 95-98 degree weather it's very hot even with open doors. As far as I can tell there is nothing in that truck that will be using much of electricity from the battery or batteries I should say (it has 2 connected in parallel) except for normal stuff (like guages and fan to cool the engine)

    Spot cooler is a spot air conditioner http://www.windchaserproducts.com/ac...d/airc_SC7.htm I could only find 115v one so I had to buy an inverter.

  5. #5
    FLAC PhilG's Avatar
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    OK, now I understand, it's an A/C solution. Thats a pretty cool idea. What is that unit worth?

    Your setup should work fine. I would suggest mounting the invertor as close as absolutley possible to the batteries and use 1/0 wire with fuse to connect. Let the 110v cooler cord make the trip to the inverter rather than the other way around. I am concerned about the quality of that invertor. $100 for a 2500w invertor is dirt cheap but I guess you will find out soon enough what it is capable of.
    My 2007 Ford F350 Work Log located HERE

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    A/C unit is $280 and inverter is 105, everywhere else it's around 200, but Amaz0n has it for 105. here . Hopefully it will work, manual says connect it directly to a battery, what fuse are you talking about? Wires that they include are (200A) #2 guage, 2 red 2 black, 6 foot each.
    Last edited by blk133; 07-20-2011 at 02:18 PM.

  7. #7
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    There should be a maximum 200A fuse at the battery end of the 200A cable - unless that cable is short and physically protected.
    That fuse is to protect the cable in case it shorts to ground, but could be halved to ~100A since you will be running under half load.

    I doubt that your inverter will be 87% efficient at 690W; efficiencies are usually quoted at ~80-90% load (ie, 1600-1800W).
    And alternator outputs are at their nominal RPM - usual not idle and low RPM.

    The best way to be sure is a dash voltmeter. If it dips below ~12.7V (or 25.4V), then your battery is discharging.
    It should normally be above 13.8V (27.6V) and up to 14.4V (28.8); lower than that means insufficient alternator output (at that load and RPM).
    The voltmeter should be across the battery(s); not ignition etc. (Vehicle/charging voltages are based on the battery.)


    Apart from distribution dimensioning (cables, fuses, switche - ie Amps), essentially is is a mere power equation.

    Alternator 100A. That's 100 x 13.8V = 1380W output (probably max at sufficient revs, hopefully long-term).
    Cooler is 690W (input).
    Assume 70% converter efficiency, 690/.7 = 985W.

    Hence an alternator headroom of (1380-985=) 395W (nearly 30%) for engine, gauges, radios, lights, fans, partner's hairdrier & iPod, and insufficient engine RPM.

  8. #8
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    Quote Originally Posted by OldSpark View Post
    There should be a maximum 200A fuse at the battery end of the 200A cable - unless that cable is short and physically protected.
    That fuse is to protect the cable in case it shorts to ground, but could be halved to ~100A since you will be running under half load.

    I doubt that your inverter will be 87% efficient at 690W; efficiencies are usually quoted at ~80-90% load (ie, 1600-1800W).
    And alternator outputs are at their nominal RPM - usual not idle and low RPM.

    The best way to be sure is a dash voltmeter. If it dips below ~12.7V (or 25.4V), then your battery is discharging.
    It should normally be above 13.8V (27.6V) and up to 14.4V (28.8); lower than that means insufficient alternator output (at that load and RPM).
    The voltmeter should be across the battery(s); not ignition etc. (Vehicle/charging voltages are based on the battery.)


    Apart from distribution dimensioning (cables, fuses, switche - ie Amps), essentially is is a mere power equation.

    Alternator 100A. That's 100 x 13.8V = 1380W output (probably max at sufficient revs, hopefully long-term).
    Cooler is 690W (input).
    Assume 70% converter efficiency, 690/.7 = 985W.

    Hence an alternator headroom of (1380-985=) 395W (nearly 30%) for engine, gauges, radios, lights, fans, partner's hairdrier & iPod, and insufficient engine RPM.
    This answer confused me even more. In short, will it work and still keep my batteries charged or it won't work? FedEx truck doesn't have radio, hairdryer or iPod (lights are on only in winter but I am not going to use A/C in winter). There is no sigarette lighter either. I need 2 of these from what I understand http://www.solar-electric.com/fb110ampfuwi.html
    Last edited by blk133; 07-21-2011 at 10:17 AM.

  9. #9
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    It might work....

    Your alternator may put out nearly 1400W at adequate RPM.
    Your inverted 690W load will probably take almost 1000W.

    That leaves at most 400W for other things and charging.

  10. #10
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    Another option is to replace an alternator with 150 amp. I assume connecting an extra battery isn't going to help much with the current set up?

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