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Old May 24, 2002, 10:31 PM
high-speed freak
opualuan's Avatar
San Jose, CA
Joined Sep 2001
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here are the weights and capacities of the cells I will be offering initially:

100ma 4.5g
200ma 6.0g
600ma 14g
900ma 29g
1200ma 34g
1700ma 50g
3000ma 80g
3800ma 108g
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Old May 27, 2002, 04:19 PM
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Turkey
Joined May 2002
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In my previous post I wrote that the M logo belonged to Mabuchi by mistake; it should be Matsu****a. The link I gave belonged to Matsu****a motor (obviously is in thePanasonic group).

Regards,

Bulent
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Old May 27, 2002, 04:27 PM
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Matsus hita has been filtered again, we must remember not to use the forbidden word..

What a bug..
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Old May 27, 2002, 04:38 PM
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Joined Feb 2002
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My experience to date with 135 mah Li-Poly is:
First run, mini direct drive prop on N20, 400 mah @3.9V, ran 20 minutes.
Second run, U-80 direct drive on N20, 820 mah @3.65V, ran 8.5 min. cell temp. rose 7 degrees.
Third, fourth, & fifth runs..HV M20 (TT), @ 5.8:1, stock Gunther 5" prop. Starting current 610 mah @ 3.8V, ran 14 min. one degree temp rise. RPM started at 3800, dropped to 3600 by 2 min. at 10 min. was still doing 3400 rpm. Conclusions: 600-650 mah initial draw seems about right, good motor runs (10+min) no real rise in cell temp. Using Gordon's figures for the Gunther, starting thrust was 18+ grams, dropped to about 15 grams @ 11 min. Now to find out what I can fly with 17-18 grams of thrust, then see how long the cells last. If temp. is any indication, stress is minimal at this current draw. BTW, charging at 100 mah takes about 1hr 15min to auto cut-off with no measurable temp rise.

Dave W.
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Old May 27, 2002, 05:38 PM
Sticky Shepherd
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Oxford/England
Joined May 2001
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This sounds good, I'm on the edge of buying a wod of these batteries. 17/18 grams of thrust is loads when you consider the weight of the pack. A 10" pitts special would be nice

Graham
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Old May 28, 2002, 12:19 PM
FLYER spelled I-squared-R
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Grand Rapids, Michigan
Joined Sep 2001
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So guys, if I can summarize (I'd like to get comments, especially opualaun's input on the larger cells), it looks like the LiPoly will give us a practical 5C drain with at least reasonable (20-30 cycles) life. Is that what I'm seeing here?

David (fugitive from the Parkflyers forum)
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Old May 28, 2002, 01:09 PM
in persuit of low wing loading
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Boston, Mass
Joined May 2001
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Dave W.

It's nice to see you getting similar independent results. These just look better and better. Good work.

To change the subject, how about how to use them in practice? Apparently the cell pictured on the Kokam site with the small circuit board is the one with cell protection. This seems to mean it prevents discharge below 3v. I'm not sure, but this seems to be all it does. This additional feature adds about $1 to the cost of the cell from the manufacturer. This is a hard cutoff, which we don't want. It should be possible to make/design something that could go between a plain LiPoly and an RFFS-100 or other receiver that would have a soft cutoff. Since an RFFS-100 operates down to 3v, a device that reduced volts once the 3v (or thereabouts) level was reached would still allow plenty of volts to run the system. It should reduce fast enough to get our attention that the plane is slowing down, but not so fast that there isn't enough power left to land the plane.

Does anyone with more technical expertise at designing such a unit know how to proceed? Is this something that could be developed and schematics published here that we could individually build? The ideal solution would be an RFFS-100 with a soft cutoff for the ESC. I'm sure the reason one was not included with the present design was (1) weight, and (2) no problems associated with NiCd's and NiMH when discharging. But, the micro world has just changed and now we need something with a soft cutoff. The alternative is to fly the plane for less time than we know it is capable of to avoid overdischarging. This is what I plan to do since the flight times appear to be quite long.

Fliir,
You might want to check out the WildRC site. For the larger cells with GWS motors Dan Krieg gives some specs for max discharge rates for the cells he is selling. But, yes, it appears we can discharge at around the 5C level so far.
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Old May 28, 2002, 01:35 PM
Sticky Shepherd
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Oxford/England
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If you wanted an add on unit it would have to sense on the input of the receiver and then take the go between the rx and the motor. The power to the receiver would have to remain constant, to reduce it you would have to use a resistive element and that's bad.

Making a LiPoly specific Rx would also be difficult as there are no spare inputs into the pic to feed a "voltage status" you could have logic gates between the pic and drive to the mosfet so when there is low voltage the drive to the mosfet is cut. This is is all weight.

Personally I think the better method is to just use time. If you know the drain of the model, you can estimate how long the charge will last, take a few minutes off and don't go below that. I wouldn't want to fly for 20 min so after a 5min stint you could just give the battery a top up.

What about just an LED to indicate 3v has been reached, you can get tiny SMT ones that weigh nothing and even in hyper bright versions. A simple circuit of two resiters and a comparitor could be used to set it off. When the landing lights come on then you, well land.

Graham
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Old May 28, 2002, 01:55 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
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Graham,
I agree that generally I don't want to fly for 20 minutes. And, there is no problem topping these off. But, I'm curious, why do you say the voltage to the receiver needs to remain constant? That's not what happens with the RFFS-100 using either NiCd or NiMH cells. The voltage declines during the flight. Why couldn't such a unit fairly quickly reduce volts from 3v to 2v, still well above the RFFS-100's 1.5v minimum. But, this would induce a dramatic slow down in motor thrust. The volts to the receiver and motor would stay at 2v while you land.

Maybe I'm concerned about nothing.
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Old May 28, 2002, 02:01 PM
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Buffalo, New York
Joined Jan 2002
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Gordon,
My dad and I were discussing this over the weekend. Indeed we do have plans to come out with an RFFS-100 (maybe the RFFS-101 ) that would effectively cut off the or reduce the motor current when the voltage drops to accomodate Lipoly cells. We are going to explore the matter further and possibly start production on it this weekend.
Unfortunatly we didn't come up with a simple light solution for those who already have an RFFS-100. WE will have to think on it further.

Dan

BTW for those who have to verbally say RFFS-100 we have come to call it roofus 100.
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Old May 28, 2002, 02:04 PM
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With my self-made DC booster/ESC/BEC combo I have chosen to make constant 5.2V from the 4.2V-2.7V of the Li-Poly cell. This voltage is fed to the RX (which can be a standard one like the GWS), the actuators and the ESC. When battery voltage attempts to drop below 2.7V I throttle back the ESC automatically so that is avoided. This is done by a feature of the used IC which contains a voltage reference and a comparator. The RX and actuator voltage remains at 5.2V which is good IMHO.

The result is that motor power is constant for a very long time (about 17min), then goes down rather quickly, giving a clear hint to land.

I agree that flying so long can be boring after the first few times (it was really amazing the first time!), but I like to have constant power and to be able do decide to land when I'm bored, not when the battery is empty.

These new small cells might be just the right stuff to have 5-10min of good power, then it's time to land.
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Old May 28, 2002, 02:28 PM
Sticky Shepherd
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Oxford/England
Joined May 2001
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Quote:
Originally posted by gordonjohnson
But, I'm curious, why do you say the voltage to the receiver needs to remain constant?
Well it doesn't, it can gradually decline but to produce a gradually declining voltage you need to use a switching regulator (?) or some sort of resistive element (half on mosfet). Considering that DU probably hunted high and low for a "low on resistance" mosfet for the speed control it would be better to avoid futher losses from another element, then there is the fact that is has to pass upto an amp so it might not be all that small. Better to just cut (or reduce with pwm) the power to the motor.

Put one of these fellows in the air and see if you can break it, if you can then we need to worry.

Graham
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Old May 28, 2002, 03:29 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
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Graham,
Ok, now that you have made me think more clearly, what we need is a device that cuts the voltage to the receiver (and hence the motor) to a fixed amount. If at 3.10 volts it cut it to 2.0 volts that would get our attention, and the device could be much simpler and probably lighter. So, how does one go about making one of those? I don't think we can cut power to just the motor since the RFFS-100 has an integrated ESC. Again, maybe we don't need one, but I'm just thinking here.

Our next indoor is June 8. I'll be flying my Stechmücke using a LiPoly and any other small planes I can get completed (my wife has a very agressive spring planting schedule for our yard and garden this year). My Dr1 seems to have passed a low-altitude glide test from about 2.5 feet up. I hope to have it done and try flying it. I'll report what I learn.
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Old May 28, 2002, 03:31 PM
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USA, FL, Fort Lauderdale
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OK Guys, how about some verbal descriptions, ie size, weight etc. as to what 15 - 18 grams of thrust will fly. I can scale my se5a plans to any size I want (currently 16" span - 90 sq in) but the weight will be detrmined afterward. I am not looking to bore holes in the sky, just some scale-like WWI flying speeds. I am also thinking that at 16" span, 1/32" for the wings may be a bit flimsy (twisty) so 1/20" may be needed. How about a bit of input?
As far as the L/V cutoff, I think fot the moment, I can live with the timing solution.

Dave W.
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Old May 28, 2002, 03:42 PM
Sticky Shepherd
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Oxford/England
Joined May 2001
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Gordon, you can cut power to the motor the cut off device becomes like a second esc, still not ideal but perhaps better.

Dave, my Se5 is 11.75" wingspan and uses two nicad cells. Therefore it's a safe bet you could fly it on one of these cells as they are lighter than the two cells combined and have a higher voltage. This would give it some what incredible performance but you could trade this in for some extra wingspan and slower flight speed. I wouldn't go bigger than 14" wingspan personally but it depends on how light you build the model.

To give you an idea of what 15-18g means to me, I have a two cell 16" wingpan high wing "power house" it uses a kpoo direct drive giving me about 7g thrust. It flies great and weights 25g. Attached is a pic.

Graham
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