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Old 07-19-2014, 06:00 PM   #1
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Default Battery C

How much lead way should I give the battery over motor amps. for a ex sample I have a motor that has a max of 40A is a 2650 at 20C is it good enough
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Old 07-19-2014, 06:31 PM   #2
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Old 07-19-2014, 07:02 PM   #3
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Originally Posted by egrave1 View Post
Hfor a ex sample I have a motor that has a max of 40A is a 2650 at 20C is it good enough
Yes that should be fine. Bear in mind that just because the motor is rated at 40A doesn't mean it will actually draw that many amps, and even if it did in static testing the motor will unload in normal flight.
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Old 07-19-2014, 07:48 PM   #4
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Generally if you get an acceptable flight time then you don't have to worry about C rating with 20C+ packs being about all you can find now.

20C = in theory you can deplete the pack in 3 minutes safely. Few people are happy with just 3 minute flights. We tend to want 6 to 15 min with some wanting even longer flights.
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Old 07-20-2014, 02:32 AM   #5
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Originally Posted by egrave1 View Post
How much lead way should I give the battery over motor amps. for a ex sample I have a motor that has a max of 40A is a 2650 at 20C is it good enough

For those who wonder what "C" represents, and how to use it, take a look:

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

DennyV
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Old 07-20-2014, 06:34 AM   #6
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I'm going to be the odd one out ....

If the motor is rated at xx Amps on paper ... often I find the motor actually pulls MORE amps than that when run at spec'd prop etc.

Second if I find a motor pulls 40A ... I would try NOT to use a 20C pack even if the mAh x C calculation gives higher capability. I have puffed too many low C rate packs doing just that.
I would prefer to have a C rate of at least 2/3 of the amp draw ... a 40A draw is not that high nowadays really .. with many models well over that.

But as you go up the scales into the 60A and higher - then because of their are few really high C rates of packs - you do have to work on that mAh x C basis.

I've now got to the stage that I do not use any pack of less than 25C ... for me - it's better to have a pack that is easily OVER the required than to be risking puffing a pack. It tends to be forgotten that the pack that is calculated to work - will often be great in winter - but come summer and it gets hot outside ... she puffs in the higher working temps.

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Old 07-20-2014, 08:31 AM   #7
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Nigen,

Relating c rate directly to amp draw makes no logical sense (i.e. 2/3 or motor amp rating). For planes that use large motors and capacity batteries this would force you into impossibly high c rates. For instance I have a motor rated for 140A (constant) / 200A burst, and it is powered perfectly adequately by 40c 5000mAh batteries. Going by your 2/3 rule I'd need 93c (constant) / 133A (burst) batteries
Also for small batteries '2/3 or motor amps' gives you a badly under rating, for instance if the battery was 1000mAh and motor was 30A rated your rule would say 20c batteries. In reality you would need at the very least 30c for that set-up.

This is because you are overlooking the effect of battery capacity. To calculate (theoretical) maximum discharge rate you need to factor in battery capacity as well as c rate, capacity is just as (probably more) important to battery discharge performance.

The calc (as I'm sure you know) is:

Max amps = c rate x mAh capacity/1000

So the OP's batteries are theoretically good for 20 x 2650/1000 = 53A (constant)

Most 20c batteries have a 30c burst rating meaning that amp draw increases (in theory) to 80A burst.

I'm like you in not thinking it's a good idea to push batteries to their limit but in this case they aren't anywhere near their limit. They will be perfectly ok, but having said that, 25c batteries are almost the norm these days so it might be worth looking around because it could well be possible to get higher rated ones for virtually the same cost. Higher c rate is never a bad thing and will cover for possible future 'hopping-up'.


Steve

PS... If it's found that the motor pulls more than it's rated 40A then the only safe thing to do is prop down, not fit a higher rated battery.

PPS... it's true that manufacturers often exaggerate c rate, but this exaggeration is greatest as high c numbers. The 20c-25c ones are usually 'reasonably' honestly rated but once you get up to higher ratings you do need to take the claims with a big pinch of salt.
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Old 07-20-2014, 08:46 AM   #8
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Hi JPF ... you must have skipped through my post ...

If you go back and read it carefully - I specifically say that at high amps you have to work on the capacity x C rate rule.

My 2/3 is based on lower amp draw installations such as up to 40A ...

>>>>
if I find a motor pulls 40A ... I would try NOT to use a 20C pack even if the mAh x C calculation gives higher capability. I have puffed too many low C rate packs doing just that.
I would prefer to have a C rate of at least 2/3 of the amp draw ... a 40A draw is not that high nowadays really .. with many models well over that.

But as you go up the scales into the 60A and higher - then because of their are few really high C rates of packs - you do have to work on that mAh x C basis.
I think you know me better ...

My reasoning for using a higher C rated pack in the lower amp draw set-ups is purely that too many packs are optimistic in their C rates ... doesn't matter whether HK or whatever label ... its what I've found. Using a 20C 2100 pack in a 40A situation has usually led me to have a puffed or extremely warm to hot pack ... particularly when summer comes along.
I don't see point of running a pack at it's limits when for just a couple euros more - you can have a pack that sits comfortably with the demand.

But get up in the higher amp draws ... some of my speed jobs are pulling over 100A when given the gun ... and yes - I agree that the C x Ah needs to be used .. as we don't have packs high enough rated to keep to my 2/3 opinion ...

OK ?



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Old 07-20-2014, 08:51 AM   #9
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Originally Posted by solentlife View Post
OK ?
Actually the 2/3 rule is even worse when applied to small setups.

Take the example i added as an edit to my last post:

1000mAh battery, 30A motor - 2/3rd 'rule' says 20c battery.

In reality the absolute minimum required is a 30c battery, but you and I would fit at least 40c to give a bit of headroom.

Try applying it to really small setups where the motor is rated for 5 or 10 amps and you get stupidly low c ratings, even though they often require quite high rated batteries.

So basically the 2/3 'rule' doesn't really work at all IMHO.
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Old 07-20-2014, 09:45 AM   #10
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Originally Posted by JetPlaneFlyer View Post
Actually the 2/3 rule is even worse when applied to small setups.

Take the example i added as an edit to my last post:

1000mAh battery, 30A motor - 2/3rd 'rule' says 20c battery.

In reality the absolute minimum required is a 30c battery, but you and I would fit at least 40c to give a bit of headroom.

Try applying it to really small setups where the motor is rated for 5 or 10 amps and you get stupidly low c ratings, even though they often require quite high rated batteries.

So basically the 2/3 'rule' doesn't really work at all IMHO.
JPF - you are attacking my view from wrong angle ...

Look at it from the overall capability AMPs figure ... in YOUR last post - I totally agree IF you are calculating your way.. but the rule DOES work when considered from the other angle.

Take a requirement to supply 40A ... and you have at hand a pack of say 2100 20C.... 2100 300C and so on ... I would use the 30C pack based on :>

My way is to calculate what pack size based on needed total amps .. ie C x Ah .... so we have 40A as req'd. Now use that as 2/3 ... this then gives a total required as 40 / 2 * 3 = 60A

Now take your packs and find the one that gives the required calculated total 60A .. ie a 3000 20C or a 2100 29C ... means as I said use the 30C pack ... then you have no problem at all ..

Nigel

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Old 07-20-2014, 12:13 PM   #11
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Simply buy the highest "C" rating you can within reason, 40C is pretty common and cheap nowadays.
You will have lower internal resistance meaning the batteries will run cooler and last much longer than stressing them near their limits.
Your esc will run cooler as well as the capacitors on the esc.
I run a lot of very high amp setups like 200+amp 4s setups, never had a problem as long as the batteries are up to the task.
Don't bother with the batteries that claim 70+C, those numbers are complete BS, in most cases you would melt the leads off the battery if you actually tried to draw the maximum current these batteries claim to deliver.
Just last week I looked at a battery that claimed you could pull 540amps through 12 gauge wire and deans connectors!
Also don't believe motor ratings for a second, the motor will draw whatever amperage it requires to turn the prop regardless of how much it is supposed to draw. I have a couple 36mm motors "rated" for 60 amps, I have the data logs showing them pulling about 120 amps. Quality motors, they are still running strong, never burned one up. Lots of cooling required as it's heat not amperage that destroys a motor.
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