Arising Star Power Issue

[Simon UK]

Hi Guys
 
I have spent this last week Electrifying my Seagull Arising Star. Anyway to cut a long story short, it had it's Re-Maiden today.
 
First i will tell you how it's kitted out.
 
Turnigy 3548-900 Motor
RC Smart 80A ESC (seems to work great and was as cheap as chips on Ebay)
5S1P A123 battery pack (made up my me)

12x8 APC - E prop

 

All up Wieght is about 6 pounds

 
My Watts-up gives the following readings

 

 

My problem is this, ROG was achieved within about 15 feet (very good i think) but the plane would lose altitude on anything less than about 3/4 throttle and was happiest on full throttle.

It was quite a windy day today in all honesty and if a good friend and very good pilot Gerry had'nt of been at the club i would have stayed grounded. This may have played a part in the power problem?

 

Is there an easy way for me to fix this? I could maybe add another cell to the pack to make it a 6s1P pack, would this be enough do you think?

Thanks for any advice given

A Big MERRY CHRISTMAS to all you Aeromodellers and your families.

[Stephen Grigg]

Yes ,put an OS46 in it

[birdy]

Perhaps you should try a higher pitch but smaller diameter prop.

[Tim Mackey]

A coupler of things Simon.....

5 x A123 cells should be showing around 16 V at least under load, so not sure why you are only showing 13V...this shows your cells are sagging to 2.6V under  load, and resultant power of just 483 Watts is poor for a 6lb model. Even as atrainer i would want to see 100 watts per pound.

At those voltages also, your RPM will be a bit low on  A123 cells as they have lower nominal voltage than Lipo and this motor is designed around 3 - 5 Lipo not A123.

First off we need to know why your cells are sagging so much, then you really need to up the watts by either adding a cell or increasing prop. I think possibly a combo of both might be needed, as it sounds like the pitch speed of the chosen prop is a bit low.

 

[David]

The poor thing is under-powered. You need to get the watts up to 600 at least at full throttle. this willl mean that the current then is at least 46 amps. Your ESC is OK with this - but your battery may not be. A 20C battery would need to be at least 2500mAh. The next problem is to increase the prop size to extract the extra power - and you may run into prop clearance difficulties - I would suggest that you will need a 13 x 8 or may be a 14 x 8 to extract the extra power.

 

However, if you go to 6s1p that would increase the revs and almost solve the problem - provided your battery capacity is adequate as above.

[Simon UK]

Stephen i just took an ASP 46 out of it, smelly oil burners are not for me im afraid :P

Going to recharge the cells and take a new reading Timbo, maybe the cheap cells from e-bay are not up to scratch

 

Tbh i was quite happy with the power and amp draw on the 12x8, i thought the numbers where fine, maybe just not for a 6lb plane.

[Tim Mackey]

The battery will, theoretically be fine at 46A, the A123s are rated at 100A+ in bursts if needed, with 70A constant being within spec.

However, duration will be an issue at these currents as A123s are only 2.3A cpacitry, so you will be pushing them at 20C which at WOT would give just 3 minutes.

I think firstly as I said, we need to find why they are sagging so much - maybe they are new and need running a bit ( I found that A123s did improve with a few cycles on them )

For duration, either make a 2p pack or higher "S" count...then of course, the motor may be too high in Kv.

Edited By Timbo - Administrator on 19/12/2009 17:14:31

[Bruce Richards]

Was the battery fully charged? Because 12.99 V seems low for a 5S pack only 2.6V per cell. When fully charged they should be 3.6V per cell. So either they are not fully charged or not holding up to the load.

 

Are the brand new? I have herd that A123 take a while (several cycles) to get to their best.

 

The other thought is to go 5S 2P.

 

Another thought is it just way over propped. If the motor is 900Kv then 17V (fully charged under load) would be trying to spin the prop over 15000rpm. May be 4S 2P would be better with this motor.

 

Lots of possibilities too much to choose from with limited information but you defiantly need more power.

Edited By Bruce Richards - Moderator on 19/12/2009 17:17:04

[Tim Mackey]

Ah, now we confess that the cells were from fleabay - in my experience, they are probably S/H and may well be poor abused cells.

Are you even sure they are genuine A123 cells from A123 systems?

[Bruce Richards]

You a quicker typer than me Timbo

[Simon UK]

They look genuine to me, they are the green plastic covered ones. Also other buyers had stated that they where happy with them so i am hoping they are fine, they apear also to be brand new so (crossed fingers) they maybe will get better with a few cycles. Tbh the first readings may have come from not fully charged cells, i am recahrging as i type. Also the plane on re-weighting comes in at 3.1 kg's 6.8lb's thanks to some dead wieght (which i hate) needed to obtain a decent cog. Another cell would at the very least help me lose most of the dead weight.

 

Lastly the 12" diameter prop is the bigest i can spin, i have just over 1" ground clearance with this on.

 

Will post new readings within the hour.

 

[Tim Mackey]

M1 A123 cells are white paper tube covered

 

I have a horrible feeling you may have bought the low capacity 1100m/a cells

Do they look like this ?

 

Edited By Timbo - Administrator on 19/12/2009 17:55:46

[Simon UK]

Ok new reading's on Full charge after 10 seconds on full power

 

 

 

I think the batteries are a bit duff! They are dropping to 13 volts from a charge of 18 hmmm. There is a slight improvement but it's neglegable. Do you think i should give them a few cycles or just buy some new ones. I see HK are selling them at half decent prices, i sincerely hope THEY are genuine and new as stated.

 

Edited By Simon UK on 19/12/2009 18:35:18

[Simon UK]

Timbo A123 do make the cells in green shrink wrap.

 

Mine look identical to these ones.

 

http://www.a123systems.com/a123/products

 

 

 

[Tim Mackey]

Ah ok, looks like the real thing then!

I take it you have done a pukka job of the soldering etc?

I would cycle them a few times before writing them off.

Incidentally how much did you pay per cell?

Edited By Timbo - Administrator on 19/12/2009 18:40:40

[Simon UK]

My soldering was so-so new to it tbh, I may have (i know i did) have to apply my iron for longer than ideal to get the positive ends to heat up.

 

I hoped i hadn't killed the cells, and they seem to charge fine, and my ballance charger tells me they are all at 3.6 fully charged or within .01 of it anyway.

 

I will cycle them a few times and see if it helps. Cells cost me about £5.50 each which is a good price (i hope not too good a price).

 

So if they improve a bit with cycling how would you suggest i find my extra 100watts of power. A new motor is a possibilty looking at this

http://www.giantcod.co.uk/xyh4260-600kv-outrunner-p-404602.html

 

would it do a better job on 5s a123's?

 

Thanks for any advice

[Simon UK]

The Turnigy SK 3548-900 was bought for a different plane about 2lb's lighter than the Arising Star. It was all i had to hand when i got fed up with I.C and the stat's told me it would fly my Star (just did'nt know how well).

 

So what do you think of the above motor on A123's ?

 

I do not have limitless funds so a decent budjet offering is the best i can do. Bearing in mind my prop can't go larger than 12"

 

 

[Tim Mackey]

Well TBH £5.50 is sounding a bit too good to be true for brand new genuine cells.

I havent bought any for about 6 months, but last lot I bought were almost £10 each

I guess the motor would be Ok, but the issue is with the battery - it simply wont provide enough power, and there must be a reason. Theres no way they should be sagging so much under a comparitevly light load.

[Tim Mackey]

The other motor you linked too is a lower Kv but will only produce the power with that cell count, by using an even bigger prop - and you cant do that. Otherwise your stuck with using much higher voltage.

I think as i said before, we need to get to the bottom of the voltage issue of your cells.

[Simon UK]

well just performed a static test, ran motor full tilt for 10 seconds then backed of to 3/4 throttle, ran motor again full throttle every 50 seconds for 10 seconds. Just to clarify

 

10 sec's full throttle 

50 sec's 3/4 throttle

 

I got 5 minutes 12 seconds of this, then the rpm and voltage droped like a brick so i stopped the test.

 

On examiniation the battery pack was quite warm, not hot but boarderline. I am letting it cool down now so i can recharge a try again. I am determined (hopefully not ignorantly) to get some use out of this pack

 

 

[Dusty]

Make it a tail dragger and prop up?

[Simon UK]

The thought had crossed my mind Dusty, but i would like to keep the amps as low as possible to give me a decent run time.

 

[Simon UK]

3 minutes flat out

 

constant 36 amp draw and constant 460 watts of power, voltage remained over 12 for the full 3 minutes then fell away drastically in seconds to 9 ish.

 

This is the 4th charge of these batteries.

 

At end of test batteries and motor where hot to the touch, is this normal? If not what would cause it?

Edited By Simon UK on 19/12/2009 23:06:27

[Simon UK]

Now maybe this will put the cat amongst the pigeons. Bellow is a graph i found on the net, while trying to figure out these M1 cells and why i'm losing so many volts under load.

As you can see from the graph 1 cell putting out 40 amps will drop to 2.6 (ish) volts

Multipy that by 5 and you get 13 volts!

 

 

 

So guys tell me if this looks right to you, would save me buying more cells to find i get the same results.

 

Lastly if i did make the plane a tail dragger and put a 13" prop on her, the amps would increase. But this would create more power, and mean i would nead less throttle therfore maybe giving me better longevity. Is there any logic in my thinking or am i of mark again?

 

Cheers

 

Edited By Simon UK on 19/12/2009 23:34:10

Edited By Simon UK on 19/12/2009 23:36:06

Edited By Simon UK on 19/12/2009 23:37:09

[Tim Mackey]

I wouldnt worry too much about the heat - the cells especially prefer to be quite warm, and work better as a result. Assuming you are happy with the cell voltage ( and I still reckon its a bit low under load - I was holding 15V under 43A load from a 4S2p Life pack in a model recently )..... the only way to get more power on the same prop is to increase the voltage, so add another cell. This will of course increase the current even more, and duration is going to start to be a problem on a 6S1P pack.

Dare I suggest 4s LiPo may be better for your situtaion? RPM will be high mind.

Edited By Timbo - Administrator on 20/12/2009 00:26:42