'C' rating . . . how much do we need?

[Clifford Stone]

I'm moving on to big birds, 3.7m Shadow to be precise, I finding the eyes are not what they used to be and my 2m e-gliders can get a bit small in good lift. The 12ft jobbies are very visable at the same 350-400m height! At 350m+ its panic time with 2m e-glider, full flaps, down elevator, a great adrenalin rush but the 'old' heart does pound!

I picked up a shadow this weekend, its been well flown but loved and has a good few wins to its name. I have to put a power train in and radio, then we are away. The motor, prop, ESC chosen is: Turnigy SK3 Power Glide 830kv, 45w, 70a SESC I'm planing on using the recommended 14x10 folding prop, (actualy I have a Graupner 14x9.5) ease the load a tad? This will be driven by 3 cell, 1800mah Nano-tech batteries 'A-type', 65c - 130.

My question, how much, if at all is 65c over kill. I have some 25-50c Nano-tech's, 2200 and 1000amh, could these be used, bearing in mind, the motor only runs for 30 seconds max, albeit at full power?

I have heard, we over estimate the 'c' draw of our motors . . . any help or comments would be appreciated, before I commit to buying any more 'high c' rated lipos? I have also been told the high c rated batteries have a short life compared with more modestly rated units . . . Not sure about that one, my Nano-techs are getting puffy and old just a year old.

CJS

Edited By Clifford Stone on 29/09/2014 19:57:12

Edited By Clifford Stone on 29/09/2014 20:00:12

Edited By Clifford Stone on 29/09/2014 20:04:27

[Dave Hopkin]

The answer if too put a meter in the power train and see what current you are drawing at full throttle - that will let you work out what C rating you need

Puffy LiPo almost always caused by over discharging them rather than the number of times they go through the charge/discharge cycle - for longevity then dont take them below 20%

[Clifford Stone]

OK Dave, I always use a watt meter to see power (w) and be sure to keep within the amp spec., of motor and ESC. But its the current bit I dont understand how to interpret?

Puffy Lipo, occasionaly I discharge to 14%, I will have to rethink, thanks.

CJS

[Dave Hopkin]

If you know the Wattage and you know the Voltage, Current can be calculated as follows:

Current = Watts / Voltage

 

So on a 3S Lipo (11V) if you are getting 200 Watts (for example) you will be drawing 200/11 or 18.18 Amps

 

 

Edited By Dave Hopkin on 29/09/2014 20:19:05

Edited By Dave Hopkin on 29/09/2014 20:20:18

[John Privett]

Posted by Clifford Stone on 29/09/2014 20:11:46:

OK Dave, I always use a watt meter to see power (w) and be sure to keep within the amp spec., of motor and ESC. But its the current bit I dont understand how to interpret?

OK, so you know the current that the motor is drawing - by using the wattmeter. Ignore (for the moment) any figures it shows you in Watts, the current in Amps is what matters. Let's say, for example, you find it draws a maximum of 66A, (as your ESC is rated at up to 70A then hopefully the actual current will be a bit less than 70A!)

Now turn to the batteries. If you're using a 2200mAh battery then 1C is the current that is numerically the same as the capacity of the battery. So for a 2200mAh battery 1C is 2200mA. Which is 2.2A. So 22A would be 10C, and 66A (who says I cheated by choosing that figure as an example!) would be 30C. And for the 1000mAh batteries, 1C is 1000mA, or 1A, and 66A would be 66C.

So you'd be pushing the 1000mAh batteries beyond their 25-50C rating, but be reasonably within the 25-50C rating for the 2200mAh batteries if you're only running for a short length of time.

[Chris Barlow]

You can also increase the power from the battery by increasing the capacity to maybe a 3000mah for the same c rating as the 2200mah.

If you need more nose weight it might as well be in the form of a bigger battery than dead lead!

It's also easier on the wallet!

[Clifford Stone]

Hi Dave, makes sence I think . . . 11v -:- by say 500 Watts = 45 That makes one sit up and take notice . . .

If I am understanding that correctly, sorry I'm not the sharpest knife in the draw when it comes to maths of any type, I have dyslexic problems with numbers (and spelling, you may have noticed) that makes things really difficult.

The theoretical watt figure is what I need or am hoping for? These big birds weigh 2kg+, near vertical is desirable? The previous owner of my Shadow tells me she weighs 2.2kg ready to fly. I might be a bit ambishus at 500 watts, if achived I need to be looking carfuly at 'c' ratings. However, seeing other biggies fly, they don't climb anything like vertical, sacrificing power for lightness, small motors turning 16" even 18" props with a large pitch through gearboxes to get efficiency, 'flying' the aircraft to height rather than dragging it there? This small motor requirement is forced by some designs having very restricted nose diameter, fortunately Shadow has a generous 44mm.

I'm hoping, the direct SK3 Glider Drive approach will do the business?

Thanks again Dave.

[Clifford Stone]

Posted by Chris Barlow 1 on 29/09/2014 23:06:12:

You can also increase the power from the battery by increasing the capacity to maybe a 3000mah for the same c rating as the 2200mah.

If you need more nose weight it might as well be in the form of a bigger battery than dead lead!

It's also easier on the wallet!

That another part of the equasion Chris. All this is currently theoretical, it is an option I will look at when she is at the check 'CG stage'.

There is also the use of 4 or 5 cell Lipos . . . they are very big birds, over 12ft, not 'fast', small props are not the best idea but it is there as a possibility? Its simply having the knowledge at hand to try as required. For instance a 4 cell 2200amh with a 12" prop might be an option?

I am told some of the really serious e-glider guys take 12 months to get a model trimmed and powered to perfection. Me, 12 days is a long time . . .

CJS

[Dave Hopkin]

Hi Clifford

Looking at the motor you mentioned **LINK** i Assume?

Its rated for 2-5S LiPo with a max wattage of 830...with a max current of 45A, but on 3 Cell you wont be seeing 830W, more like the 500 ish you mention.... that should be enough to fly it to altitude - after all its not meant to prop hang is it!

And some thing like a 1800Mah should give you a decent enough power run without undue dead weight for the main event of the flight, the gliding bit......

After that it will come down to playing with props to find the best on for it........

If you wanted to increase the length of time of the power run a battery with a bigger Capacity (MAh) would do the job- at the cost of more dead weight of course....

[Clifford Stone]

Posted by John Privett on 29/09/2014 22:52:17:

Posted by Clifford Stone on 29/09/2014 20:11:46:

OK Dave, I always use a watt meter to see power (w) and be sure to keep within the amp spec., of motor and ESC. But its the current bit I dont understand how to interpret?

OK, so you know the current that the motor is drawing - by using the wattmeter. Ignore (for the moment) any figures it shows you in Watts, the current in Amps is what matters. Let's say, for example, you find it draws a maximum of 66A, (as your ESC is rated at up to 70A then hopefully the actual current will be a bit less than 70A!)

Now turn to the batteries. If you're using a 2200mAh battery then 1C is the current that is numerically the same as the capacity of the battery. So for a 2200mAh battery 1C is 2200mA. Which is 2.2A. So 22A would be 10C, and 66A (who says I cheated by choosing that figure as an example!) would be 30C. And for the 1000mAh batteries, 1C is 1000mA, or 1A, and 66A would be 66C.

So you'd be pushing the 1000mAh batteries beyond their 25-50C rating, but be reasonably within the 25-50C rating for the 2200mAh batteries if you're only running for a short length of time.

Mmm, getting a bit confused, lets take facts from an existing 2m e-glider I have. I gives almost 300 watts of power on the meter, ignore that. I draws 20 amps, and uses a 1000mah 3 cell battery rated at 25-50.

My dyslexic mind says that 1c x20= 20c, batter working with in its constant 'c' rating . . . ?

Theory of new model:

Motor is rated at 45amh max, ESC is 70amh, proposed battery is 3 cell 1800amh 60-130c

1c = 1.8A, 10c = 18a, 45amh is 4.5x18 = !!!

If my understanding is correct, I'm looking to keep the amps drawn to about 35?

Raises the question how far can one encroach into that burst figure . . . if at all. The motor run is 30 second max.

This might be telling me why they use small motors?

CJS

[John Privett]

Posted by Clifford Stone on 30/09/2014 00:32:36:

Mmm, getting a bit confused, lets take facts from an existing 2m e-glider I have. I gives almost 300 watts of power on the meter, ignore that. I draws 20 amps, and uses a 1000mah 3 cell battery rated at 25-50.

My dyslexic mind says that 1c x20= 20c, batter working with in its constant 'c' rating . . . ?

Yes, 1C for that battery is 1A. So 20A that the motor draws is 20C for that battery - well under its 25-50C rating.

[Clifford Stone]

Posted by Dave Hopkin on 30/09/2014 00:18:31:

Hi Clifford

Looking at the motor you mentioned **LINK** i Assume?

Its rated for 2-5S LiPo with a max wattage of 830...with a max current of 45A, but on 3 Cell you wont be seeing 830W, more like the 500 ish you mention.... that should be enough to fly it to altitude - after all its not meant to prop hang is it!

And some thing like a 1800Mah should give you a decent enough power run without undue dead weight for the main event of the flight, the gliding bit......

After that it will come down to playing with props to find the best on for it........

If you wanted to increase the length of time of the power run a battery with a bigger Capacity (MAh) would do the job- at the cost of more dead weight of course....

Hi Dave that it, the whole thing is theory at present, although I have all the hard ware as yet uninstalled. I'm simply concerned I don't over stress anything . . . although in the competition environment one will take things to the limit.

As you say, props are the final factor to get it right . . . 30 seconds of climb to 200m with the aim to 'glide' for 10 minutes and land on a sixpence! (an unaided, no thermal flight is 4 to 5 mins max). Am I mad or what . . . perhaps I'm kidding myself, as I said, I'm wondering if the other pilots use small motors with gearboxes to address this 'power' issue?

I always do things different, its this strange dyslexic mind I have, approach a problem from a totally different angle than normal, cos' I'm not normal, tell me it cant be done . . . I will find away.

CJS

[John Privett]

Posted by Clifford Stone on 30/09/2014 00:32:36:

Theory of new model:

Motor is rated at 45amh max, ESC is 70amh, proposed battery is 3 cell 1800amh 60-130c

1c = 1.8A, 10c = 18a, 45amh is 4.5x18 = !!!

If my understanding is correct, I'm looking to keep the amps drawn to about 35?

OK, so the motor is fine for up to 45A. The ESC is rated at 70A so will be fine at the 45A limit for the motor.

45A is only 25C for that battery. (For that battery 1C = 1.8A, so 45A = 45/1.8 = 25C) As the battery is rated at 60-130C then 25C is well within its capabilities.

[Clifford Stone]

Posted by John Privett on 30/09/2014 00:57:57:

Posted by Clifford Stone on 30/09/2014 00:32:36:

Theory of new model:

Motor is rated at 45amh max, ESC is 70amh, proposed battery is 3 cell 1800amh 60-130c

1c = 1.8A, 10c = 18a, 45amh is 4.5x18 = !!!

If my understanding is correct, I'm looking to keep the amps drawn to about 35?

OK, so the motor is fine for up to 45A. The ESC is rated at 70A so will be fine at the 45A limit for the motor.

45A is only 25C for that battery. (For that battery 1C = 1.8A, so 45A = 45/1.8 = 25C) As the battery is rated at 60-130C then 25C is well within its capabilities.

OK got it . . . 1c is 1/10 of the battery capacity, divide that into the amps drawn shown by the watt meter in this case 45a -:- 1.8 = 25c . . . that makes sence. Sorry if I have re written what you have posted John, but it put it in my dyslexic mind in a way 'I understand'. I have discovered over my 67 years that I see things differently by translation, so the rewriting or repeating my way in the case of face to face, translates a description so that I understand . . . Doh!

You are all very helpful and patient guys, thank you.

CJS

[Clifford Stone]

That was very good and informative.

We have establishes that a fairly standard 25c-30c battery will serve my needs. What advantage, if any, would it be to use 40c or even 60c rated batteries, obviously their burst rate will be considerably higher but I am looking at the continuose rating as practical.

CJS

[Ed Anderson]

Lots of good information here so I will not rehash. BTW I am also an e-glider pilot. I fly a Radian and an E-Supra for ALES contests in the USA.

In reference to C rating, the closer you run to the packs maximum rated amperage the harder you are working the pack and the faster it is likely to degrade.   Also, the closer you run to the max rated amperage the more the voltage drop which will translate into a slower climb.

You can't have too high of a C rating. But you can have too low, as has been stated above.

I usually try to keep my top amp draw at or below 80% of the maximum sustained C rating in my gliders. I typically ignore burst numbers.

My e-Supra weighs about 68 ounces all up with 1300 mah TP 65C 3S pack.   It pulls about 45 amps for 24 seconds to reach the specified 200 meters for the typical ALES launch. I use the 65C rated Thunder Power 1300 mah 3S pack most of the time. That battery is rated for 84 amps but I am only pulling 45 so I am not even working the pack hard.

BTW, each launch, followed by a 10 to 20 minute flight uses about 30% of the pack's capacity, about 400 mah. For contests I swap packs after each flight. For sport flying I will take 1 full climb and two 15-20 second reclimbs for about an hour's flight on a pack. This consumes about 70% of the capacity. I don't want to take it any lower than that but there is enough reserve to get me a short burst if I need it to save the glider.

I have also used Thunder power 850 mah 70C 3S packs to try and shed some weight. That pack is rated for 59 amps so I am running at about 76% of the sustained max C rating. I get about the same launch time but only do one full launch on these packs as that climb plus the flight is pulling about 400 mah. That leaves me enough reserve to do a restart if I need it to save the glider but I typically don't do reclimbs on these. They are mostly for contest work where a restart would get me a zero for the flight.

Hope that practical experience is helpful.

 

Edited By Ed Anderson on 30/09/2014 14:21:30

Edited By Ed Anderson on 30/09/2014 14:23:14

Edited By Ed Anderson on 30/09/2014 14:24:11

[Clifford Stone]

Hi Ed, very useful personal experiences that. It about sums up how I try to do things, and the sort of battery packages I have.

Your fun/practise flying is very similar to mine when on the field by myself. However, there are half a dozen of us in my club who fly the competition type e-glider. We get to the field as often as we can as a group, all retired so that is as regular as the weather and 'she who must be obeyed permit' We spend 3 or 4 hours in friendly competition with each other, full fit out, watches, spot and all, great fun, excellent company and ones flying improves, I think? The banter is good and I always enjoy the lunchtime pick-nic. These semi serious sessions highlight any problems in technique, model trim and we draw on each others experience. I still haven't don much good at the competitions, first season so lots to come, there are a couple in October then its all over until next spring.

Be flying in 2m and Open class next year, been working on Shadow today, servos are in, found a small ding, waiting for the glue to dry. Then its programing the Hitec Aurora 9, A daunting task with my dyslexic problems, but I get there in the end, just takes time and patience. If I get a problem my mates at the field always help.

Thanks again for your experience, CJS

 

Edited By Clifford Stone on 30/09/2014 16:13:04

Edited By Clifford Stone on 30/09/2014 16:13:57

Edited By Clifford Stone on 30/09/2014 16:14:14

[John Privett]

Posted by Clifford Stone on 30/09/2014 01:21:53:

OK got it . . . 1c is 1/10 of the battery capacity, divide that into the amps drawn shown by the watt meter in this case 45a -:- 1.8 = 25c . . . that makes sence.

Just one small correction there...

1C is the battery capacity, not 1/10 of it. (Or rather, it's the same number, the units are slightly different.) So for your 1800mAh battery, 1C is 1800mA. And 1800mA is just another way of saying 1.8A.

The rest of your statement is correct though. When the current is 45A, then the number of C is 45/1.8, which is 25C.

[MattyB]

A caveat... Let's not forget that 99% of pack C ratings are vastly inflated, with many leaking into the totally made up category. Of those I have used only Gens Ace seem to be realistic; the HK batteries are particularly optimistic. Don't believe me? Measure the internal resistance of your packs and calculate the true C-rating using this tool - be prepared for a surprise or two...