Underperforming Lipos or something else ?

[Steve Colman]

I'm flying a Sebart Shark with the following:

Motor: Emax GT2820/07, 850Kv, 12x6 prop

ESC: Generic, 50A constant, 60A burst

Lipos: Polypower 3S 2650mah, 30c

After a while just tootling around, I've just started exploring the aerobatic capabilities of the model and although it performs well it seemed to lack any real punch.

Checked the setup with a watt meter today and was shocked to find that the power system was only producing 260W at wot. On a 4S lipo the motor specs indicate a tad more than 600W. Of course I didn't expect anything like that power on 3S but 260W does seem a bit below par.

My suspicion is that the lipos are not delivering anywhere near their stated C rating.

Your knowledgable thoughts much appreciated.

Steve.

[crispin church]

you are hopping a lot from a 30c 2650mah

is that the largest battery you can fit in ?

have you tried different props ?

are your batterys balance charged test with meter what to

and remember c rating is what the company say they are so can be taken with a pinch of salt

[Allan Bennett]

All the info I can find on that motor seems to indicate that on 3S it can only take 26A, whereas on 4S it can handle 41A. That's illogical to me -- quite often motors have a maximum watt limit, but that would entail less amps for more volts.

Anyway, taking their figures as gospel, 26A at 3S would give you a maximum of 327 watts; you can only get anywhere near 600 watts by using 4S at 41A (688 watts).

Personally I would increase your prop diameter until your wattmeter tells you the amp draw is around, say, 35A (because their maximum amps is a burst rating, not continuous), which will give you another 100 watts to play with. Then check the motor temperature after a short run -- too hot to hold is too hot, so reduce the amps by reducing prop size.

I doubt that your LiPos are the problem, unless they're really at the end of their life. For a 2650mAh battery to deliver 41A (much more than you're using at the moment) it only needs to be working at around 15C, so even if the 30C rating is optimistic, it can't be that bad.

[Chris Bott - Moderator]

Steve what are your voltage and current readings? From the voltage we can see if your batteries are indeed dropping off because of the load.

I've just been experimenting with an 850KV motor. On 4S with an 11x7 I'm seeing 750W 50A 15V

Of course different 850kv motors will be a little different.

[Andy48]

The Emax 2820 should use an 11x7 propellor and draw a max of 38A with a 3S. See manufacturers spec: **LINK**

If it still gives a low wattage, then possibly something wrong with the Lipos, or maybe a bad connection. Maybe your Lipos are getting a bit tired, and the internal resistance is rising fast.

I can confirm a 4S with a 12x6 will draw just over 50amps and give a thrust of over 2kg.

Edited By Martin Phillips on 06/01/2013 20:09:17

[Chris Bott - Moderator]

Posted by Martin Phillips on 06/01/2013 20:08:36:

I can confirm a 4S with a 12x6 will draw just over 50amps and give a thrust of over 2kg.

But only if the LiPo's are up to it.

[Andy48]

I was being conservative, a new Lipo will give over 2.2kg of static thrust with a 2820. Believe me I've been doing quite a bit of research on this motor of late...

[Steve Colman]

Many thanks for the prompt replies gentlemen.

More info which I forgot to add earlier. In addition to the 260w reading on the meter it also indicated a current draw of 40A ! From this I assume that upping the prop size wouldn't be sensible on 3S?

The Lipo's have only had a dozen cycles from new and after balance charging are very close to 4.2v per cell. In terms of specs; size, weight, etc they are similar to those recommended by Sebart for the model.

After a typical flight of 8 minutes battery consumption averages around 1400mah, the lipos are warm but not hot. Similarly, the motor and esc are warm to the touch but no more.

Tomorrow I hope to test again using a 4S lipo from a friend.

Many thanks again gents.

steve.

[Steve Colman]

Martin, The motor is the newer GT version which has slightly different specs.

[Chris Bott - Moderator]

Steve getting 260W at 40A indicates a pack voltage of 6.5V under load.

Something is clearly wrong, probably with the batteries.

Go carefully with 4S, the results will be very interesting though.

[Steve Colman]

Chris, my thoughts exactly.

My intention tomorrow is only to do a static test with 4S. The other obvious thing is to test some different 3S lipo's.

[Simon Chaddock]

I may be a bit 'old world' but to me Watts only tell part of the story, its the Volts and the Amps together that give the full picture.

As Chris has pointed out if you are really drawing 41A and only showing 260W then you probably either have already killed the LiPo (it won't like being pulled down to 6.5V) or there is something wrong with your meter!

[Martin Harris - Moderator]

The Wattmeter I have is woefully inaccurate so don't assume the readings are necessarily correct. The one thing not mentioned so far is the air temperature at this time of year which can impact seriously on battery performance. Have you been doing the testing at the field or with batteries at room temperature?

[John Privett]

Steve, something doesn't add-up with those figures! Does the wattmeter give you a voltage reading as well as the amps and watts? Because the watts figure should just be a multiple of volts and amps, and as Chris said the figures you quote mean a battery voltage of 6.5v!

That aside, my "gut feeling" is that your prop is somewhat on the small side. I was flying my Sebart Katana 30E this afternoon. Sebart's recommendation for this, and for your Shark, is a Hacker A30-14L motor with a 14x7 prop. The Hacker is an 800 Kv motor. I have a Scorpion S3020-16 motor (812Kv) in the Katana with the recommended 14x7 prop. That has "adequate" power (I can prop hang and pull long verticals) and get about 6 minutes using PolyPower 3S 2200 mAh 20C batteries. (I also have a Turnigy battery of similar spec and don't notice any difference in performance between them.)

So, I appreciate your motor has a slightly higher Kv than both my Scorpion and Sebart's recommended Hacker, but you're running a much smaller prop. I think that's why you're drawing rather less power than you want it to. You say you're taking 1400mAh out of your battery after 8 minutes. I usually put back about 1500-1600mAh into the battery after a 6-minute flight. I'd try a bigger prop!

[Steve Colman]

Once again, thanks for your replies guys.

Now a confession. I feel somewhat embarrassed to say that I got my figures completely muddled.

Re-testing today the figures were 260W , 26A on 3S and 600W , 40A on 4S. Thus, confirming the figures that you guys have stated given the motor specs. and also confirming that my lipo's are fine.

Please accept my apologies, I do feel somewhat foolish.

I will test some bigger props to see if I can eek out a little more power however.

Steve.

[PatMc]

Max current for the motor is 41A according to this chart so you should be able to get about 450W from 3s, just a question of prop size so lon as the battery can maintain 11v at 15.5C give or take.

[Steve Colman]

Thanks for the link to the chart PatMc. Very useful info.

[Andy48]

Steve. Sorry for getting the wrong motor. Doh!

However if you look at PatMc's link and the one I gave above from the EMax website, you will find that the GT2820 /07 will give 41amps on a 4S battery, but 26amps on a 3S. Your answer probably would be a 4S battery. I have tried various propellers on the BL2820, and found that the Emax specification of a 12x6 proved the best. Interesting though, with a brand new Lipo, with a really low internal resistance I could only get 51amps not the 59 as stated.

I have yet to try fiddling with the ESC timing setting. This may make a difference. Failing that, if you cannot fit a 4S, you could change to a GT2820/04 and get a little more power. Just a thought.

[Chris Bott - Moderator]

Steve it's just good to know everything is OK. Thanks for letting us know.

[PatMc]

Martin, if Steve increased the prop size as John Privett suggested he should be able to increase the current to 41A for a power draw of circa 450W using 3s.

[Andy48]

Not entirely sure about this, but I seem to remember reading that the maximum efficiency of a brushless motor is at about a third of maximum load. Found the above chart for a brushless motor which would seem to indicate that too, and also show that increasing the torque load (ie prop size/pitch) does not always lead to more power.

Emax are very specific about prop size, I would assume that this the most efficient size, and a larger prop will draw more current but may not give an increase in performance.

I do know from some of my tests that the efficiency is higher at lower currents. I must experiment more with a few bench tests. Hmmmm.

[PatMc]

Martin, the efficiency isn't relevant to what is required here, the aim is simply to have more power on tap. Increasing the load until the max safe current is obtained will take the max electric input power & in practice result in the max output shaft power.

Sorry but without any source references or quantifiable parameters your graph is merely baseless opinion that TBH is pretty vague & meaningless.

[Andy48]

Nooo! It isn't as simple as that. Can I quote from the Emax GT box "The motor efficiency is higher when it is run before maximum current specifications."

What happens is that as the maximum current is approached, the efficiency starts to drop off and actually you could well have less output power, though be using more current and draining the battery faster, which is exactly what the orange curve on the very relevant graph above shows.

[John Privett]

Martin, without any figures on it, the graph is of academic interest, but of little practical use. For all we know in the real world it might be impossible to run a particular motor in the part of the graph beyond half-way along the "torque load" axis without it "letting out the magic smoke." Indeed it looks like that would mean the motor would be turning at about half the speed indicated by the Kv rating - so way over-propped and probably well and truy dead (see this thread for an example...)

It seems likely (but without any figures on the graph, difficult to prove) that Steve's motor is running way to the left of the grey "Rated operating point" line on the graph. Increasing the load will increase the power output until the load gets to extreme values.

 

Edited By John Privett on 08/01/2013 22:28:29

[PatMc]

Martin, the Emax statement only says the efficiency is higher below max current. It doesn't say that the output power is greater. If we only ran our motors at the highest possible efficiency we would have a lot of underpowered models.
I've linked to this page in Model Motor's (AXI) site to illustrate the difference between efficiency & higher power output. Look at the example performance table at the bottom of the page. You will see that running a 10x5 on 10 RC2000 cells produces an output of 224.5W 27.3A for 76.2% efficiency whilst running a 12x6 on the same cells increases the output to 300.9W & current to 40.5A but drops the efficiency to 71.6%.
Now look in the specification table you will see that the max efficiency current is between 20 - 30A but the max current is 42A/60secs.

The max safe operating current for any motor is not the stalled current, which is one reason that the graph is meaningless. For all we know the orange curve could be reaching it's peak at the point of the max safe operating current. However I'm sure that the safe current will have passed before the max power point is reached.

Getting back to the thread topic, if the GT2820/07 motor can be run safely at 41A on 4s then it can certainly be ran at 41A on 3s. The input will be down by 25% & I would expect the shaft output to be down by a similar proportion. However on 3s the prop dia would be greater thus probably more efficient than that used on 4s so the actual available power would probably not be reduced by the same proportion.

PS I typed this up before I saw John's post but we seem to have reached similar conclusions.

Edited By PatMc on 08/01/2013 22:48:53