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  1. #1

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    Help me choose a battery please!!

    So I'm new to electric. That forum on this website for electric training helped out quite a bit. I'm sick of the mess of nitro and the cost of nitromethane. SoI bought my first plane that i plan on converting to electric. It's a fun aero superjake. It's technically going to be a conversion to electric as the plane calls for a .71 nitro engine, but I don't think its anything I can't handle. I do however need help choosing the right size battery. I plan on running 3s, butI'm not sure how many amps I should go with. The brand doesn't really matter to me it's flight time I'm after. Here is the motor and ESC I plan on using. It's going to be a little overpowered because this plane according to my numbers is going to weigh somwhere in the neighborhood of 2 1/2 pounds fully loaded up with the battery and everything. I'm on a budget soI'm using some rather generic products(I've mostly been looking for batteries at hobbypartz.com if that helps)

    http://www.bphobbies.com/view.asp?id...7&pid=B1898545
    http://www.bphobbies.com/view.asp?id...2&pid=B2049175

    Any help would be much appreciated
    Thanks


    Danny

  2. #2
    Moderator aeajr's Avatar
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    RE: Help me choose a battery please!!

    Welcome to electric flight.

    The first place I would like to point you is:

    EVERYTHING YOU WANTED TO KNOW ABOUT ELECTRIC POWERED FLIGHT - A free e-book
    http://www.rcuniverse.com/forum/m_7100376/tm.htm

    If we target a 2.5 pound plane, all up, then you want at least 250 watts for spirited sport flying. So that motor looks good.

    The chart tells you EXACTLY what the amps will be based on battery pack voltage and prop, so figuring that out is easy. Assuing a 10X5 prop you will pull 28 amps at full throttle.

    A 3S 2800 mah pack will give you about 5-6 minutes of full power flight or about 9-12 minutes of mixed throttle. Is that enough? Go up from there.

    Don't forget to include the weight of the battery in your calculations of all up weight. Also consider the space where the battery will go.
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  3. #3
    Glacier Girl's Avatar
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    RE: Help me choose a battery please!!

    Like aeajr said!

    And here's a simple formula for figuring flight times at wide open throttle using 80% of the pack capability. 80% pack usage is the rule of thumb for keeping the pack happy, and providing long life.

    Divide the pack capacity by maximum amp draw, multiply that by .8, then multiply that by 60. That will give you what the pack will safely provide in minutes of wide open flying.
    Note at reduced throttle the time will increase, but this does give you a good idea of where you stand flight time wise.

    Like this we'll say using aeajr's specs above, a 2800 mAh pack and a 28 amp draw system.

    2800/28000 = .1 .1 x .8 = .08 .08 x 60= 4.8 So your pack will fly the plane, at wide open throttle, for 4.8 minutes.
    Missing Man Formation, Looking heavenward you can't help but shed a tear, mournful, lonesome, a hole that screams out at you.

  4. #4
    Moderator aeajr's Avatar
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    RE: Help me choose a battery please!!

    Glacier Girl provides good advice.  That formula works.  I only did an estiamte in my head.

    Note that her calcultions are based on bench readings which is what you would expect if you put that prop on that motor on that voltage.  However there is an "unloading factor" that happens in the air.  Your actual amp draw at full throttle will be about 15-20% lower than your bench numbers.  I just ignore that in my calculations as it provides the same kind of battery margin that .8 provides in Glacier Girl's calculations.

    Most people don't fly at full throttle all the time.  That is where the mixed flying numbers come from.
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  5. #5
    kzart's Avatar
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    RE: Help me choose a battery please!!

    I learned something very important today about this from an experienced 3-d flyer...He came up with this equation 10 grams of battery weight = 40 watts of power. I was assuming that my 2100 mah batterys would b more than enough for a plane that recomended an 1800 mah battery( omega 72 setup). i flew my plan and it was somewhat underpowerd. So, I started asking a bunch of questions (almost in a panic because I thought my plane was stuck this way). But my 2100 mah battery was only producing about 300 watts of power while a heavier 1800 mah battery will give closer to the recomended 500 watts for my plane. I also assumed that a 4 cell would be stronger. But, my new best friend explained that cells= volts and volts are used to adjust prop size. i wish there were a place that gave and explained these formulas....

  6. #6
    Moderator aeajr's Avatar
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    RE: Help me choose a battery please!!

    go to electric  general  forum

    Free book

    Everything you wanted to know about electric .flight

    Sticky thread at tip of forum.
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  7. #7

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    RE: Help me choose a battery please!!

    I just bought a zippy 8s compact lipo from Hobby King. I have other lipos and all of them have one balance lead. The zippy 8s has two balance leads and my charger, a Turnigy Accucell 8150 will balance an 8s but only an 8s with one lead. So, my question is how do I charge and balance the zippy 8s? Can I use a blinky on one of the leads and plug the other lead into the 4s port on my charger? Or should I use a blinky on both leads? I am really confused about this and I want to go about this in a safe way. I REALLY need help here.

  8. #8
    Jetdesign's Avatar
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    RE: Help me choose a battery please!!

    You need a balance port with (2) 4S inputs:
    http://epbuddy.com/index.php?main_pa...&products_id=8
    Joe Marri
    Enjoying all things aviation.


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