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whats the limit??

Old 01-03-2006, 04:09 AM
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LECTROFLYER
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from I understand you can run these lipos at thier C rating, but what about after that, meaning if a bat has cap. of 2100 and the C rating is only 15 and the motor draws 35A= 31.5........ looks like a 3.5 point diffrence, so can you push it to 3.5 points over to 35 or only use the burst rating
 
Old 01-03-2006, 04:13 AM
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Twmaster
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Never exceed the limits of the battery for any sustained draw. Burst is just that. A short burst.
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Old 01-03-2006, 10:00 AM
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rocket_jim
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Originally Posted by LECTROFLYER View Post
from I understand you can run these lipos at thier C rating, but what about after that, meaning if a bat has cap. of 2100 and the C rating is only 15 and the motor draws 35A= 31.5........ looks like a 3.5 point diffrence, so can you push it to 3.5 points over to 35 or only use the burst rating
I understand C. Yes, a 2100 MaH pack at 15C would be 31.5 amps max. And typically that's burst, a few seconds, possibly 10 seconds at a time. Maybe more, but not a lot more time than that.

I understand that props "unload" in the air some, and that people don't generally fly at 100% throttle constantly.

But what I don't understand is people contemplating hooking up something that at full throttle would drain the battery in 60/15 = 4 minutes. That's not much flying time.

And if something goes wrong, the throttle gets stuck full-on and the plane crashes, that suggests you have less than 4 minutes to get there to shut it off or risk ruining the pack. So I don't understand why many people would even care about a 15 C pack. 8C is good enough for me. Clearly there is a class of flying way beyond the lazy, have a peaceful time flying slowflyer class that I fly.
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Old 01-03-2006, 02:59 PM
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electriconly
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It's my understanding of how a ESC works from the Castle Creations forum that if a motor/prop pulls 35amps that that is what the ESC gets from the battery.... it just switches the current on/off (PWM frequency) for shorter or longer periods of time based on the throttle input.

Now what I don't know is if at 50% throttle the ESC is pulling 35 amps 50% of the time and 0 amps 50% of the time how that relates to the "C" continuous rating of a Lipo pack.

Brad
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Old 01-03-2006, 06:06 PM
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Originally Posted by electriconly View Post
It's my understanding of how a ESC works from the Castle Creations forum that if a motor/prop pulls 35amps that that is what the ESC gets from the battery.... it just switches the current on/off (PWM frequency) for shorter or longer periods of time based on the throttle input.

Now what I don't know is if at 50% throttle the ESC is pulling 35 amps 50% of the time and 0 amps 50% of the time how that relates to the "C" continuous rating of a Lipo pack.
I follow that point (never really thought about that implication before) but we can make some estimations if we assume the C rating is dominated by thermal effects:
1. Constant current - thermal power = (Ic^2)xR
2. Pulse current (50% duty cycle) - thermal power = (Ic^2)xR/2
Therefore the 50% pulse is equivalent to Ic/sqrt(2), or,
50% pulse at 35A is equivalent to a 35/1.414 ~ 25A continous discharge

In other words at a 50% duty cycle on 35A a pack should be selected that is able to withstand a 25A continuous discharge.

HTH

Rod
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Old 01-03-2006, 09:26 PM
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electriconly
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Originally Posted by rocket_jim View Post
I understand C. Yes, a 2100 MaH pack at 15C would be 31.5 amps max. And typically that's burst, a few seconds, possibly 10 seconds at a time. Maybe more, but not a lot more time than that.

I understand that props "unload" in the air some, and that people don't generally fly at 100% throttle constantly.

But what I don't understand is people contemplating hooking up something that at full throttle would drain the battery in 60/15 = 4 minutes. That's not much flying time.

And if something goes wrong, the throttle gets stuck full-on and the plane crashes, that suggests you have less than 4 minutes to get there to shut it off or risk ruining the pack. So I don't understand why many people would even care about a 15 C pack. 8C is good enough for me. Clearly there is a class of flying way beyond the lazy, have a peaceful time flying slowflyer class that I fly.
If you look into the posibility of converting a 40 or 60 glow airframe to electric then a 15C to 20C pack becomes very attractive.

Say you want a 10 minute flight (6C avg) have a power system that draws 50 amps and uses about 3000 mah for the ten minute flight. (these are real numbers from my e-Ultra Stick 60)

You could use (20) GP3300 Nimh (as I do now) or (2) 20C 4S1P 3300 Lipos in series (8S).

With the Lipo's you lose about 16 oz in weight and gain thrust through higher voltage. Can you envision a vertical rocket ship.

Brad
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Old 01-03-2006, 09:42 PM
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Matt Kirsch
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Another advantage to the higher-C-rated packs is the reduced voltage depression under load. The pack is discharging more efficiently, and is putting more energy into turning the motor rather than heating the inside of your airplane. Running an 8C 2000mAh pack at 6C versus running a 20C 2000mAh pack at 6C, you'll get longer flight times and better performance out of the 20C pack.
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