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Old Aug 26, 2007, 10:10 PM
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Joined Mar 2007
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Question
What exactly is the difference between the LiPo C ratings?

Say for example that i have three 7.4v 800mAh Lipo's one is 10C one is 15C and the third is 20C (these rating would be the continuous ratings)

What would the difference be for each lipo pack if i used them on the same plane. will the 20C flight be faster? Does it use up the batt faster?

Just trying to understand what the ratings are all about, Thanks!
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Old Aug 26, 2007, 10:58 PM
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Joined Apr 2004
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800mah: supplies 0.8 amps for one hour.

10c: should be able to supply 8 amps for 6 minutes (10 x 0.8 = 8.0) (60/10 = 6).
15c: should be able to supply 12 amps for 4 minutes (15 x 0.8 = 12) (60/15 = 4).
20c: should be able to supply 16 amps for 3 minutes (20 x 0.8 = 20) (60/20 = 3).
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Old Aug 26, 2007, 11:20 PM
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Quote:
Originally Posted by rick.benjamin
800mah: supplies 0.8 amps for one hour.

10c: should be able to supply 8 amps for 6 minutes (10 x 0.8 = 8.0) (60/10 = 6).
15c: should be able to supply 12 amps for 4 minutes (15 x 0.8 = 12) (60/15 = 4).
20c: should be able to supply 16 amps for 3 minutes (20 x 0.8 = 20) (60/20 = 3).
Ahh, I see thanks for explaining. Am I correct in saying that the more amps provided the more power output? Does it spin the motor faster? (Sorry, I don't know a lot about electric flight, yet )
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Old Aug 26, 2007, 11:53 PM
I havent decided yet
Indiana
Joined Jun 2006
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when you think of a batteries capacity think of it like this.... the motor/prop pull the current(amps). if they are pulling current within the batteries capacity (mahX C rating) then increasing the capacity or c rating wont effect anything. the battery just "allows" the motor to pull the current that the voltage, kv of motor, and prop size determine. if the battery cant stand up to the demand then a higher rated battery will perform better. ..... in the example listed in post #3 if the motor was drawing 5 amps then in theory all the batteries will perform equally.
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Old Aug 27, 2007, 12:46 AM
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USA, OR, Damascus
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Your 800mah battery is like a bucket full of amps.
The "c" rating is like a hole in the bottom of the bucket.
More "c" means a bigger hole, means more flow
A 7.4 volt motor-prop that draws 20 amps at wide open throttle.
your 800mah 10c, 15c, 20c batteries will only give your motor what they are able, but not enough for that motor.

A 20 amp motor needs a bigger battery bucket.
1000mah 20c 20 amps for 3 minutes
1500mah 15c 22.5 amps for 4 minutes
2000mah 10c 20 amps for 6 minutes
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Old Aug 27, 2007, 01:06 AM
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Quote:
Originally Posted by Ramen
Ahh, I see thanks for explaining. Am I correct in saying that the more amps provided the more power output? Does it spin the motor faster? (Sorry, I don't know a lot about electric flight, yet )
Volts spin the motor - determine RPM. The higher rated packs will experience less voltage sag (drop) for a given load, therefore more RPM. Make sense?

Bill
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Old Aug 27, 2007, 02:07 AM
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Letchworth, Great Britain (UK)
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Quote:
Originally Posted by Ramen
... Am I correct in saying that the more amps provided the more power output? Does it spin the motor faster? (Sorry, I don't know a lot about electric flight, yet )
Yes, but all else being equal it's the motor and prop that determine how many amps the motor wants. As rick.benjamin has said, the battery is only a source of volts and amps, and it may or may not be able to meet the amp demands of the motor/prop The C rating when multiplied by the battery capacity (mAh divided by 1000) should tell you how many amps the battery is able to supply, so you can decide if that's sufficient for your particular motor/prop combo.

The battery voltage directly determines how fast the motor will spin -- it's the motor's kv rating (rpm per volt) multiplied by the battery volts.

The battery voltage and the amps being drawn by the motor combine to give you the power -- watts = volts x amps
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Old Aug 27, 2007, 03:10 AM
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Staffs, UK
Joined Nov 2003
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Quote:
Originally Posted by Ramen
Say for example that i have three 7.4v 800mAh Lipo's one is 10C one is 15C and the third is 20C (these rating would be the continuous ratings)

What would the difference be for each lipo pack if i used them on the same plane. will the 20C flight be faster? Does it use up the batt faster?
If the plane used low enough current that the 10C (8A maximum) battery could handle it you would see very little difference using the other (potentially) more powerful batteries.

The battery rating only tells you the maximum current you COULD take. It's the motor and prop that decide how much current is actually being used.

If your motor and prop were only drawing say 6-7A all the batteries would be fine, the 15C and 20C ones would be having a very easy life.

If your motor and prop were tring to draw say 10-11A the 10C battery wouldn't do it but the 15C (12A max) and 20C (16A max) batteries would.

Steve
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Old Aug 27, 2007, 05:23 AM
A man with too many toys
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From my experience itís all about heat buildup and battery durability. Higher C batteries run cooler, last longer (more cycles before), and have slightly more power.
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Old Aug 27, 2007, 06:30 AM
PLD
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Just to make things more abusing though --- 'C' ratings are really a very coarse, thumbsuck measure of goodness knows what. The 'C' rating of a TP pack is quite likely rather different to the same 'C' rating on another pack. There's no fixed consensus or standard to base it on (other than that the battery doesn't completely fail).

What I'd really like to see is manufacturers quote the /internal resistance/ figures for the cells (from DC up to about 2kHz).

Internal resistance is very useful because it gives you a way to determine how much heat your packs are going to end up dissapating internally as well as knowing what sort of voltage drop to expect when you pull a certain load/current.

eg, If your pack has an internal resistance of 0.01R (10 milliohms), then when it's delivering 30A to the motor/ESC it's generating P = I * I * R watts of heat internally (in this case, P = 30 x 30 x 0.01 = 9W ( that's quite a LOT of heat) and it's also dropping a voltage of V = I * R which gives us V = 30 x 0.01, 0.3V (so your 4.2V cell now is reading 3.9V).

So, you can see that knowing the internal resistance is actually quite a bit more practical off hand (if you know the simple math) than the 'meaningless' C ratings.... C ratings remind me of the MIPS ratings that CPUs used to get

Hope that helps some people.

Paul.
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Old Aug 27, 2007, 11:27 AM
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It's great to be a part of RCGroups! Everyone here is very helpful, thanks for all the info. I understand more about the ratings now.
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Old Aug 27, 2007, 11:54 AM
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Now that you understand the theoretical meaning of C rate, the practical side is that many C rates are grossly overstated. A battery labeled 20C may not meet that spec or maybe meet it for only a couple of flights before failing. My suggestion is to fly short flights at maybe 1/2 the rated C and check the batteries temperature. If it's cool, you can bump the amp draw up with a larger propeller. If it's hot, reduce the draw.
Remember, the longer you fly at a given amp rate the hotter the pack will become. The temperature just keeps going up so keep a check on the pack temperature till you feel comfortable with the amp draw that pack can give you.

See this graph of temperature rise at constant discharge level.
http://www.wattflyer.com/forums/atta...9&d=1134427965
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