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Old Aug 23, 2011, 10:09 PM
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United States, NH, Newmarket
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Help!
11.1 lipo pack but plane at full power drops it to 10.3v? is that ok?

I just finished building my twins plane and i put telemerty in it and when i use anything over half throttle it reads out that the battery has dropped to the low 10's in volts and it is pulling around 36 amps but the battery is 1300mah and 20 and 25c. is this ok for the battery or not and if no then what should i do? i am kinda new to electric planes and lipo's. thanks for help.
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Old Aug 23, 2011, 10:21 PM
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What brand is it? I think you will kill that battery fairly quickly.

As a comparison i am using a 45c nano tech pulling 30 amps and it comes down at ambient temp. If i did the same with a 20c then i suspect it would be quite hot. I did measure the voltage drop between a 20c and a 45c and there was about a volt diffrence that also gives me another 1000~ rpm

http://www.hobbyking.com/hobbyking/s...Lipo_Pack.html.

Others may disagree but i generally take the C rating and divide by 3 and not excede that. The most i would pull from your pack is about 15 amps

If you were to pull 36 amps constant on that pack you would get a flight time of 2.1 minutes. Mabey you could consider a bigger battery
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Old Aug 23, 2011, 10:25 PM
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but the esc is a 20amp so how is it pulling more than that is it cus its a twin engine?
is it both esc amps added together?
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Old Aug 23, 2011, 10:33 PM
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It's pulling more because the motor wants it. The ESC does not limit the Amps flowing thru it.

You need to SIZE your ESC based on all the other parts and the Amps they pull.

Based on your readings you need at least a 40A ESC but I'd give some safety factor and go with a 45-50A.
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Old Aug 23, 2011, 10:36 PM
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OK, so you have two motors and esc's You should check the current being pulled by each motor/esc. to make sure that they are both pulling simular amounts.
Lets assume that they are even, 36/2 = 18amps. 18amps is close and you should ensure that your esc receive enough air to cool them.

How big is your plane?
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Old Aug 23, 2011, 10:36 PM
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that was the recommended esc for the motors. but does the c rating for the batt. need to be the total of both esc added together?
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Old Aug 23, 2011, 10:39 PM
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You are on the fast track to starting a lipo fire.

The ESC has nothing to do with how much current the motor will draw. That is determined by the motor and prop. A 20A ESC will just as easily run a 60A load, but only for a little while before it fries.

Since this is a twin, I assume you've got the ESCs wired in parallel and you have the wattmeter plugged into the battery behind both ESCs. In this case, you would see the total current being drawn by the system. 36/2 = 18A, so you're just barely within the ESCs' ratings.

HOWEVER, the lipo's C rating of 20 gives you a total rated maximum of about 26A (20C x 1.3Ah = 26A), which could be true or it could be BS. Going much above this will damage the battery and could make it start on fire. This is also the reason the voltage is dropping so low; the high current draw is making the voltage sag a whole bunch.

You need a pack rated for at least 36A, and preferably more like 45 or 50. A 1300mAh 30C pack would theoretically do it, just barely. A 30C 1500mAh would be better, and a 30C 1800mAh would be pretty safe.
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Old Aug 23, 2011, 10:50 PM
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it is the full size foam The Twins. the motors are KMS kbm32/21 that pull 13amps, and esc is hk 20a esc
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Old Aug 23, 2011, 10:54 PM
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Yep, esc won't limit current at and a lipo fire is possible.

We really need to know how bing the plane is so we can recomend something for you. If you can answer the following it will make it easier

Wing length
Fuse Lenght
weight
Model
Motor
Esc
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Old Aug 23, 2011, 10:55 PM
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Quote:
Originally Posted by TP16 View Post
A 30C 1500mAh would be better, and a 30C 1800mAh would be pretty safe.
Even at 1800ma he only gets a flight time of 2.3 mintues.
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Old Aug 23, 2011, 10:57 PM
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Quote:
Originally Posted by coastieflyer View Post
it is the full size foam The Twins. the motors are KMS kbm32/21 that pull 13amps, and esc is hk 20a esc
Hang on 13 amps???? Where did you get that figure?

Do you have a watt meter? e.g. below
http://www.hobbyking.com/hobbyking/s...dProduct=10080

****got it****
OK, your motors are pulling 18 amps so you might consider propping down. so if your running an 9x6 you should try an 8x6. if your motors are getting hot than you will damage them and eventually it will die. How quickly depends on how how and how often you fly.
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Old Aug 23, 2011, 11:09 PM
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this is what i have for specs.
wingspan: 57"
length: 43"
aprox weight: 24oz, 680g
servos: 4 9gram, 1 24gram
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Old Aug 23, 2011, 11:10 PM
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prop 8x3.5
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Old Aug 23, 2011, 11:21 PM
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this is my motors http://www.globalhobby.com/public/gallery/108025.asp
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Old Aug 23, 2011, 11:23 PM
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The Kv on those motors is the problem. 1750Kv motor of that size on 3S with 8x3.5 props will definitely end up drawing a lot of current.

The 13A that they claim is a maximum that you have to stay within; the motor's actual draw will depend on the prop you put on it.
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