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Old Aug 09, 2012, 07:29 AM
advanced n00b
xfatdannx's Avatar
United States, NE, Omaha
Joined Feb 2012
995 Posts
Help!
Question about C rating and what i need for my plane.

Okay it has been explained to me:

C rating x mah
1000

equates to the max amps your battery can handle.

Do you try to follow an 80% rule like with an ESC...ie my mustang pulls 36 amps, my esc should be 45-50amp to give it room.

I am building an EDF that will have the same power plant as the habu 2 by HH...have not been able to put it on a watt meter but it should not pull 80amp...is a 30c 3300mah 4s enough or should i go with a higher c rater to give it that 80% rule?

Thanks in advance guys. i appreciate all the info/suggestions you are willing to provide.
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Old Aug 09, 2012, 08:21 AM
Registered User
Staffs, UK
Joined Nov 2003
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I like some overhead in the battery capacity but I wouldn't stick rigidly to any 80% rule.

But anyway, provided "not pull 80A" means less than 80A, isn't a 3300mAh 30C battery already there ? 3300 x 30 / 1000 = 99 and 80% of that is 79.2 .

Steve
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Old Aug 09, 2012, 09:23 AM
RC Helicopter Pilot
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United States, CA, Westminster
Joined Mar 2010
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The 80% rule has nothing to do with what you are talking about here.
It is for discharging your battery during use so you don't over discharge it and kill it.
Completely different than sizing a battery pack based on current draw.
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Old Aug 09, 2012, 11:32 AM
Registered User
Gustavus, OH
Joined Oct 2007
490 Posts
Keep in mind that as batteries age, the C rating decreases. This is due to the increasing internal resistance. So, I'd opt for a higher C rated battery pack to begin with to get a longer life span.

That rising internal resistance is the main reason older packs come back warmer than they used to when used the same way. That electrical resistance is turned into heat in the battery pack.

Cliff
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Old Aug 09, 2012, 12:38 PM
Tim Lampe; Hobbico R&D
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Champaign, IL
Joined Dec 2009
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The question is under what condition does your Mustang pull 36A? Is this a measurement taken on the ground at full-throttle off the initial charge of a battery? If so, your motor will never see this much current in flight (due to the battery discharging and some unloading in air). Then, you could “sneak by” nicely with a 35A ESC, but even here, as you stated, maybe a 40A ESC would be a better choice. How much more “head room” you want to give the ESC over what the motor pulls I guess depends on your confidence in the ESC.

As for your EDF, how much more current you want your battery to be above the current draw is probably a matter of personal preference and any experimentation you might have done. Me, for my “high performance” pylon racers that pull about 70A – 80A in-flight I like to double the battery capability over the motor draw (in my case 2200mAh 65C = 143A).

For most average sport flying most of the time I think guys will tell you a nice 20% to 30% “overage” is desirable. But you will probably be flying your EDF at full-throttle most of the time and demand high performance, so then I’d lean closer to a 100% “overage” in battery capability over Amp draw (like my pylon racer example). Your 30C 3300mAh battery is rated to be good for 99A (3.3A x 30). That’s a little low for my comfort level if you’re pulling 80A in an EDF (unless it’s just a “throw away” foamie or something). So for your 80A current in an EDF I’d want something closer to the 140A - 160A range requiring a 40C – 50C battery.

I’m not saying your 30C battery is no good and that you have to get a 40C – 50C battery, but if you really value your new EDF and also wish to get full performance (and make it a little easier on your battery) I’d consider a higher C-rating battery than 30C. 30C is usually pretty “pedestrian” and normal for your regular sport flyer planes, but I think you’ll be demanding a little more than that from your battery.

Tim
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Old Aug 09, 2012, 01:06 PM
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xfatdannx's Avatar
United States, NE, Omaha
Joined Feb 2012
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Quote:
Originally Posted by TheWoodCrafter View Post
The 80% rule has nothing to do with what you are talking about here.
It is for discharging your battery during use so you don't over discharge it and kill it.
Completely different than sizing a battery pack based on current draw.

i know the rule i am talking about has nothing to do here. but to prevent wasted money on batteries before i destroy them i was looking to gather more info so i know what to get.


Tim, Cliff, and Steve. Thank you guys that answers my question. Again i am new to EDFs and just didn't know if there was such a rule of any kind people liked to follow.
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Old Aug 09, 2012, 01:49 PM
KK4NOP
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United States, VA, Virginia Beach
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Quote:
Originally Posted by xfatdannx View Post
i know the rule i am talking about has nothing to do here. but to prevent wasted money on batteries before i destroy them i was looking to gather more info so i know what to get.


Tim, Cliff, and Steve. Thank you guys that answers my question. Again i am new to EDFs and just didn't know if there was such a rule of any kind people liked to follow.
If you are flying the exact same power system as the HH Habu (and Habu 2) you would be much better off going with a 5s setup to lower that amp draw.
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Old Aug 09, 2012, 10:41 PM
Tim Lampe; Hobbico R&D
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Champaign, IL
Joined Dec 2009
3,900 Posts
Quote:
Originally Posted by xfatdannx View Post
i know the rule i am talking about has nothing to do here. but to prevent wasted money on batteries before i destroy them i was looking to gather more info so i know what to get.


Tim, Cliff, and Steve. Thank you guys that answers my question. Again i am new to EDFs and just didn't know if there was such a rule of any kind people liked to follow.
Certainly!

What I would do if I were you is scour the Habu thread. I know it's a very popular model and sure there is a long thread with much discussion about batteries. You could even do a search for "LiPo" or "batteries" in the thread.

Tim
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