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Old Jul 10, 2014, 08:46 PM
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Lipo C rating formula

OKay : saw this formula and now think I have it sorted, please confirm one way or the other

11.1 volt 2000mAh -10C
2000 milliamps = 2 amps
2 Amps x 10 = 20 amps continuous discharge

So therefore if I have a motor that draws 50 amps then I should have a battery witha C rate of 25 or more

ie 3300mah 20/30 C

As you will probably gather I dont know alot about electricity but still find it intresting
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Old Jul 10, 2014, 09:27 PM
Southern Pride
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Haralson County GA. USA
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Most C rating are marketing hype and thus if you really need 20C continious it is best to get atleast 30c if not40C. Also realize that a true 30C discharge rate means less than a 2 min. flight.A 10C average discharge rate equals a 6 min . flight but as you should only use 80% of the capacity closer to 5 min. flight.
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Old Jul 10, 2014, 10:30 PM
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There is no formula as some 25C lipo's will outperform 45-90C ones.

Also some 20 or 25C packs will perform ok and deliver their rated current for the few cycles, but then quickly drop off shortly thereafter..

Best thing to do is to research and find a high quality long-lasting lipo.
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Old Jul 11, 2014, 03:13 AM
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Quote:
Originally Posted by Crusty One View Post
So therefore if I have a motor that draws 50 amps then I should have a battery witha C rate of 25 or more

ie 3300mah 20/30 C
Your calculation is basically correct and I think you understand what you're getting. But just to be clear, it doesn't mean you need a C-rate of 25 or more to get 50A because it also depends on the size of the battery.

3300mAh at 20C would be good for 3.3 x 20 = 66A
5000mAh at 15C would be good for 5 x 15 = 75A etc

That's assuming you can depend on the vendor's C-rating which as already said is a bit debatable.

Steve
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Old Jul 11, 2014, 08:26 AM
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And, operating a pack at it's "advertised" C-rate greatly decreases the cycle life. If it's just to cover full throttle bursts you are probably OK with your calculation, which by the way is right on the money. It's the Manufacturers C-rating that is in question, not your calculations.
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Old Jul 11, 2014, 12:26 PM
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Originally Posted by hoppy View Post
And, operating a pack at it's "advertised" C-rate greatly decreases the cycle life..
Do you have any data to back that up?

I have some Gens Ace 25C packs that I regularly run over their rated output and they are holding up better than other much higher rated packs.

I think what the point here is (and it's been made several times in other threads), you cannot make any hard and fast calculations of what a lipo can deliver based on it's packaging.

ie: A 3000mah 40C labelled packed is not guaranteed to give you 120Amps sustained.
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Old Jul 11, 2014, 12:39 PM
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Even if a LiPoly is rated 25C and can be dischrged at 25C without kiling it right away it still will hae have it's ife reduced cmpared to what it would have done i fs chargeat ony say 15C. Most like to get more than minute flights thus in fact discharge ther LiPolys at less 15C aveage dischage rate.

Here is a post where I discharged a 3S 50C 2200 at up to 110Amps. , it did not die right away and during a flight which was discharge #64 I forgot I was fying a 2200 in an aircraft I normaly flew a 3300 in and discharged it to less than 10V resting and it still lasted over 100 flights .

http://www.rcgroups.com/forums/showp...0&postcount=47
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Old Jul 11, 2014, 02:06 PM
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As with most of this Chinese stuff, specs tend to be pulled out of thin air and have little relation to the actual capabilities of the product. I've seen the exact same item advertised by different sellers with different specs at times.

That's why you want to be fairly conservative with the C rating. I usually like to get double what the actual max draw will be. Also for high amp draw planes it's a good idea to get reputable brand packs with honest ratings when you can find them.
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Old Jul 11, 2014, 02:20 PM
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Quote:
Originally Posted by nighthawk0077 View Post
Do you have any data to back that up?

.
See Discharge Current Rate paragraph, page 7 here.

Hyperion stated that packs operated at the max continuous rate would yield 50 cycles or less.

An older report on discharge rate vs cycle life. http://www.researchgate.net/profile/...11eaf53c3d.pdf

Some anecdotal evidence from the electric vehicle forum - http://endless-sphere.com/forums/vie...p?f=14&t=34239

See item 6. here.

Check the temp/capacity/cycle chart here. If you agree with me that higher discharge rates create higher pack temperatures you can see how the capacity losses are related to pack temperature and subsequently cycle life.
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Old Jul 11, 2014, 02:38 PM
A man with too many toys
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Quote:
Originally Posted by Crusty One View Post
OKay : saw this formula and now think I have it sorted, please confirm one way or the other

11.1 volt 2000mAh -10C
2000 milliamps = 2 amps
2 Amps x 10 = 20 amps continuous discharge

So therefore if I have a motor that draws 50 amps then I should have a battery witha C rate of 25 or more

ie 3300mah 20/30 C

As you will probably gather I dont know alot about electricity but still find it intresting

The true C of a LiPo can be calculated.
1. Use the discharge calculator to get the max discharge current. http://www.jj604.com/LiPoTool/
2. Then divide the max discharge current by the capacity in amp hours.

I have used my Power Lab to measure IR of my new packs and the computed C comes very close to the advertised C. This was for some 25C and 35C packs that I got from hobby king. I have not tried it on and very high C packs.


This is very useful because it tells you how much you can safely draw from an old pack that has lost some of its punch.




.
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