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-   -   What's the effect of changing lipo C rate? (http://www.Wattflyer.com/forums/showthread.php?t=73983)

9thumbs 06-27-2014 07:56 PM

What's the effect of changing lipo C rate?
 
I don't understand how when changing the C rate on a lipo battery it will effect the flight time and amperage draw. If I stay with the same battery capacity and motor shouldn't I get the same flight time and amperage draw? But according to online electric flight calculators changing the C rate effects the flight time and amperage draw. Please explain, thank you.

fhhuber 06-27-2014 08:09 PM

C rating affects the battery's ability to deliver current without heating. In essence its a measure of battery internal resistance.

Higher C means lower internal resistance and less heat (wasted energy) at the same power.

For our purposes there will be very little difference seen with 20C and higher rated batteries until you are going for some higher performance aircraft. Since 20C is the lowest C rating that is commonly available, you don't have to worry about C rating until you progress to high performance models and even then the 20C might be adequate.

I run 20C 5000 mah packs (3 X 4S for 12 or 50.4 V full charge) pulling 105 amp peak and 60 amp average with the plane in my Avatar pic and its just fine. Dropping in 30C packs I only see a marginal power increase... just 1 mph difference in top speed.

hayofstacks 06-28-2014 02:40 AM

Exactly. lets say you have a 20c 2200 3 cell and your only pulling 20 amps with the motor. the battery is rated for 44 amps, so you will see very little if any differance stepping up to a 25 or 30c battery, unless the specs are just made up. which does happen more then you'd think.

9thumbs 06-28-2014 02:56 AM

That's what I was thinking. Thanks.

kyleservicetech 06-28-2014 03:02 AM

Quote:

Originally Posted by 9thumbs (Post 951605)
I don't understand how when changing the C rate on a lipo battery it will effect the flight time and amperage draw. If I stay with the same battery capacity and motor shouldn't I get the same flight time and amperage draw? But according to online electric flight calculators changing the C rate effects the flight time and amperage draw. Please explain, thank you.


Maybe this will help?


"C" and what it is
http://www.wattflyer.com/forums/showthread.php?t=65869

solentlife 06-28-2014 05:45 AM

Only thing I will add to above ...

Many specifications shown for models by sellers / factory I find to be lower than optimum for the model. A good example is in EDF's ... not only pack size is inadequate - but the C rating is restrictive. ... the 50mm EDF for example. Stock advised is 800mAh 20C 3S ... it flies but motor never gets what it wants ... Up that to a 25C and even better if you can up size as well to 1500mAh .... the model comes alive.

Basically there is a minimum that a motor needs to 'breathe best' .... think of like a car - amount of fuel it can take and the air intake size ...

Final point - it's fine that we can take C x capacity and say xx amps max draw - but if you do that - the poor old battery is really having a dogs life of it. It doesn't help that many battery packs are overstated on C rating as well.

Nigel

pizzano 06-28-2014 08:40 AM

The "C" rate or "burst" as it's commonly referred to, does make a little more difference with heli's and multi rotors. the calc's and descriptions previously mentioned still hold true.....but the need for higher rated "c" values in heli/multi rotor can make a huge difference in performance.

Here's a good link that includes a calc table......that does not just apply to multi rotor...

http://multicopter.forestblue.nl/lip...alculator.html

JetPlaneFlyer 06-28-2014 10:23 AM

Yeah, I'll second the above comment about helis. I really notice the difference between high and low c rate LiPos when flying helis, not so much with fixed wing though.

solentlife 06-28-2014 11:19 AM

With Fixed Wing ... it usually depends on type of model .. with high KV and small but high pitch props - often the C rate will need to be high. My 3700 + 4.1x4.1 pusher T45 will devour any pack of less than 35C .. A 25C will come down hot and puffed .. and performance is lacking .. put a 35C and she's away !!

Nigel

dahawk 06-28-2014 01:09 PM

I "C" said the blind man to his deaf wife as he picked up his hammer and saw. LOL

I've looked at "C" as the rate at which the milliamps are delivered. Thus, given a true C rating, a 3000mah 4S 30C will provide slightly longer flight times than a 3000mah 4S 40C. Is that correct ?

The latest edf I bought suggested a 6S 4000mah 50C . I went with a 6S 4000mah 40C as the cost was 20% less. A lot of marketing going into these I think.

-Hawk

JetPlaneFlyer 06-28-2014 02:39 PM

Quote:

Originally Posted by dahawk (Post 951675)
I've looked at "C" as the rate at which the milliamps are delivered. Thus, given a true C rating, a 3000mah 4S 30C will provide slightly longer flight times than a 3000mah 4S 40C. Is that correct ?

Not really, the lower c rate battery would only last longer if you actually used less power while flying. The power you use isn't usually controlled by the maximum that the battery can deliver but by the how hard you fly. On the other hand the high c battery might give longer flights because if over stressed the low c battery might hit LVC.

Generally though the c rating doesn't make any significant difference to flight time.

dahawk 06-28-2014 05:03 PM

Thanks for the explanation Jet ! It's easy to get drawn into lipo marketing. Puffery if you can excuse the pun.

xmech2k 06-28-2014 06:05 PM

Dahawk, I think of it like this, mAh is the size of the fuel tank, C is how fast the fuel can come out of the fuel tank safely.

solentlife 06-28-2014 06:25 PM

Quote:

Originally Posted by dahawk (Post 951689)
Thanks for the explanation Jet ! It's easy to get drawn into lipo marketing. Puffery if you can excuse the pun.

mmmm yes !

And lets be hnest if you were a LiPo factory - retailer ... you'd LOVE the paranoia that surrounds the item.

I'm all for safety and awareness of hazards ... but I cannot help but think that sales of LiPo must be higher than is necessary because of fears and paranoia.

I've just been posting on a thread where a guy shorted his battery leads and it scorched a patch of the heat shrink wrapper .. so he now has a small area of foil showing ... It's a classic 'scorch-hole' you see in heat shrink if a soldering iron or other hot touches ...

Of course safety means monitor it closely ... make sure it's safe to use .. but why does he have to bin it as some would advise ?

OK - of the soap-box !!

C rating Mr dahawk .. Hi guy - yes back with you !! I'm surprised that you posted that earlier about increased run time ..
My way of looking at run time and ratings ..

If a pack is 2000mAh 20C and I draw 20A .. then I get 6 mins total to zero.

Lets now use a 2000mAh 40C pack and still drawing 20A .. I still get 6 mins total to zero.

The only time it makes any difference is when the pack is pushed to limits and it physically cannot supply the amps demand.
The 50mm EDF's are prime example. If you use the factory recc'd 800 20c pack .. then your motor just cannot get the amps it wants .. the pack physically cannot punch that high ... But if you fit a 800 40C pack - NOW the motor can get what it demands and the pack will give shorter run-time.
Yes the C rating is part of it - but it's a pack that is despite factory specs - wrongly sized for the job. And it's an extreme example.

If we go up to the 1500 size packs in same 50mm EDF .. and we stay above 25C ... then it makes no difference whatsoever what C rating we use 25C or 40C .. you will get same run-time. Because all packs are now working within physical capability.

:rolleyes:... ;-)

Nigel

hayofstacks 06-29-2014 01:03 AM

Any power system that is capeable of exceeding the battery's "c" rating will benefit from a bettery battery. a quad or helicopter are capeable of using.quite a bit of power. pretend you have 4 450 motors on a quad pulling 20 amps each. you would need a battery capeable of 80+ amps to keep the battery from degrading. there is also much more loss due to wire lengths, connectors and all the other splices and esc efficencies.

however, my way over powered slow stick pulls 22ish amps. you won't notice any differance going from a 30c 2200 to a 40c 4000mah battery, even though the 40c '-battery is good for 1600watts, the motor is still only pulling 22.


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