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Old Dec 18, 2004, 04:31 PM
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Shot Li Po's

Hi,

I'm running an Axi 2208/34 Jeti 08 Advance controller and APC 10x4.7 prop. After 20-30 cycles both the 2 cell 340 and 2 cell 700 Kokam packs won't take a charge.

This is a fairly generic set-up, I was wondering if anyone else has suffered similar problems....
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Old Dec 18, 2004, 04:43 PM
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Walled Lake, MI, USA
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According to The Great Motor Test, at 7.0V the AXI 2208/34 with APC 10x4.7 draws 9.7A. What do you suppose is the maximum current rating on your two packs?

http://www.flyingmodels.org/motortes...xi_2208-34.htm
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Old Dec 18, 2004, 05:53 PM
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Quote:
Originally Posted by julianb
Hi,

I'm running an Axi 2208/34 Jeti 08 Advance controller and APC 10x4.7 prop. After 20-30 cycles both the 2 cell 340 and 2 cell 700 Kokam packs won't take a charge.

This is a fairly generic set-up, I was wondering if anyone else has suffered similar problems....
I aso ruined several Lopos with this setup due to believing the advertising hipe and bad judgment.
My testing AXI 2028/34 APC SF 3S1P 10X3.8 9.3V-12.3A-114W-6,300 RPMs.
2S1P 7V.-8.3A-58W-5300 RPMs.
I posted this in the past and will provide link if I can find it.

Added: Found link.
http://www.rcgroups.com/forums/showp...95&postcount=5

Charles
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Old Dec 18, 2004, 06:03 PM
40%, 5% whats the diff?
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Baltimore Greenbelt, Maryland, United States
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I'm using a tp 860 2s and a 11x3.8 cut down to 10x3.8. I have had about 70 cycles I got the pack in march and it still running strong! I gues the 340's dont put out enough umph, and the etecs certainly would not.
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Old Dec 18, 2004, 06:18 PM
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Quote:
Originally Posted by everydayflyer
2S1P 7V.-8.3A-58W-5300 RPMs.
Charles, your figure of 8.3A @ 7V with a 10x3.8 prop is pretty good verification of the 9.7A @ 7V with a higher-pitch 10x4.7 prop from The Great Electric Motor Test chart.

While the Kokam 340s were ahead of their time as the first 20C LiPo, they are also very small, so 20C x 340mAh is just 6.8A.

The Kokam 700s are only rated for 6C, or 4.2A.

Any questions about what 8.3A or 9.7A loads will do to 4.2A and 6.8A packs?
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Old Dec 18, 2004, 06:33 PM
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Any questions about what 8.3A or 9.7A loads will do to 4.2A and 6.8A packs?
--------------------------
No but I do have a question as to why this combination is advertised by at least two nationally reconized hobby shops who have been in the e flight business for a great while. I admit that I should have checked amp draw before flying this setup but I could not find my shunt, my Astro Flight Watt meter had been borrowed and not returned and they were on back order and I was brand new to Lipos.
I found the link and posted above. Somewhat surprised no one tried defending the referenced vendors befor ,perhaps this time.

Charles
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Old Dec 18, 2004, 08:17 PM
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Not sure why I didn't respond to that previous message, Charles, as there is a "defense" for the vendors: Throttle management.

Some people with some aircraft are able to successfully use Li packs with lower current ratings than the motor/gearing/prop combo draws at full throttle because they rarely use full throttle for more than a second or two. The packs will handle such burst current draws briefly, but not for extended periods. Thus it is critically important when using such a combination to have a very sensitive touch and timing with the throttle.

Unfortunately, some people purchase these combos without fully understanding how critical throttle management is to making the battery packs live. Perhaps the vendor didn't do a good job of explaining, the consumer didn't do a good job of researching, or a combination of the two. The purpose of using an undersized battery pack is to make the aircraft as light as possible for aerobatic maneuvers. The downside is that the user cannot freely use full throttle as can be done when the motor's maximum current draw does not exceed the battery pack's rating.

In any case, I'm always skeptical when I read a message from someone complaining that the Li pack on a combo capable of drawing more current than the pack is rated for "failed." In most cases, it is the user who "failed" to understand that the pack will only live if it is babied with careful throttle management.
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Old Dec 18, 2004, 08:52 PM
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Kok 340

In the same way that a perfectly good glow engine can be ruined with one run of a minute or less with too lean a needle setting, a perfectly good pack of any battery can be ruined by placing a big overload on it. We have provided LIPOCALC II at www.fmadirect.com to help you avoid the kind of problem you created by choosing too small a pack. I have seen good Ni Cd and Ni MH packs that were run so hot that the heat shrink on the cells split.

In the same way, we have seen large numbers of car and airpllane ESCs ruined by putting far too great a load on them. It is important that the pack and speed control be matched to the need.

The biggest single probelm for electric flyers is that most have no way of knowing what their motor is drawing. Thus, they really don't know what Amps figure to plug in to LIPOCALC. If in doubt, we recommend being conservative. In your application, a poster has indicated that the set up draws 9.7 amps, that is almost 150 % the maximum rating for a KOK 340. That the KOK 340 or any other cell in that size range lasted 25 cycles is a testament to the cell. Now that you know what the current drain is, a quick run throught LIPOCALC II will show you the proper pack to use. You can check out the KOK 910 by pugging its spacs in in the custom pack set up at the nbottom of LIPOCALC II.

Investment in a decent ammeter would save the cost of a couple of ruined packs. Unfortunately, a decent ammeter that will measure even 10 amps is a tad steeper than we al like. FMA is working the problem and will soon offer the lowest cost setup that you can imagine to measure up to even 100 amps using an inexpensive ammeter. I once started a thread in this forum for people who made measurements to report them for as many set ups as we could get so that you can be guided at least roughly. The thread was not a sticky and drifted off the page almost immediately. Perhaps Micahel can make a sticky that people can use to report hundreds of set ups for reference. We have introduced an inexpensive, tiny on board recorder and now are doing beta test of a microammeter that you can plug in line to record flight profiles as well as static current draw. Watch the web site for announcements. Then you can check for yourself to see what throttle management really does. The new 4th gen HDR packs will be in soon and you can pop a 910 mah in your birdie that will live a long life at up to 18.2 amps max. Running it in your airplane at 9 amps will impose a load of just 10 C and the pack will deliver the full 910 mah capacity. Projected life under that condition will be several hundred cycles. That is what HDR is all about.

Happy holidays,
Fred
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Old Dec 18, 2004, 09:03 PM
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Haralson County GA. USA
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I somewhat agree Dave and yes good throttle management is important.My average times with my 3D models are obtained by using an average amp draw of around 35-45 % of max. however the very deceiving ad claim(at least to me)cut n paste from Hobby Lobby ad.
Flight times exceed 15 minutes with Lithium batteries..
is just unreal.In order to keep that combination in the air,much less do and manuvers,3D or otherwise would require using an average of only 1.36 Amps.and that is assuming that you get all 340 ma out of the pack,fat chance.

.340*60/15=1.36Amps average drain(amp. draw ) during flight.
1.36X7.5V=10.2 watts. How many really believe that is enough power for fly an IKARUS Shock Flyer?
OK they said with Lithium batteries not specificily the 340 mA ones which they recomend with the total recommended package but to me it is still very poor advertising policy.
Many beginners turn to Hobby Shops for guidence with there new purchases and with suggestions on a pcakage deal such as this no wonder so many give up on electric flight.
Remember in my case electric flight was not new to me only Lipos and even I was mislead.
By the way it turned out that the Kokams were only 3 or 5C rated(unmarked) but defenitely not the 20C cells.

Charles
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Last edited by everydayflyer; Dec 18, 2004 at 09:24 PM.
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Old Dec 18, 2004, 09:12 PM
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Quote:
Originally Posted by electroman
In the same way that a perfectly good glow engine can be ruined with one run of a minute or less with too lean a needle setting, a perfectly good pack of any battery can be ruined by placing a big overload on it. We have provided LIPOCALC II at www.fmadirect.com to help you avoid the kind of problem you created by choosing too small a pack. I have seen good Ni Cd and Ni MH packs that were run so hot that the heat shrink on the cells split.

In the same way, we have seen large numbers of car and airpllane ESCs ruined by putting far too great a load on them. It is important that the pack and speed control be matched to the need.

The biggest single probelm for electric flyers is that most have no way of knowing what their motor is drawing. Thus, they really don't know what Amps figure to plug in to LIPOCALC. If in doubt, we recommend being conservative. In your application, a poster has indicated that the set up draws 9.7 amps, that is almost 150 % the maximum rating for a KOK 340. That the KOK 340 or any other cell in that size range lasted 25 cycles is a testament to the cell. Now that you know what the current drain is, a quick run throught LIPOCALC II will show you the proper pack to use. You can check out the KOK 910 by pugging its spacs in in the custom pack set up at the nbottom of LIPOCALC II.

Investment in a decent ammeter would save the cost of a couple of ruined packs. Unfortunately, a decent ammeter that will measure even 10 amps is a tad steeper than we al like. FMA is working the problem and will soon offer the lowest cost setup that you can imagine to measure up to even 100 amps using an inexpensive ammeter. I once started a thread in this forum for people who made measurements to report them for as many set ups as we could get so that you can be guided at least roughly. The thread was not a sticky and drifted off the page almost immediately. Perhaps Micahel can make a sticky that people can use to report hundreds of set ups for reference. We have introduced an inexpensive, tiny on board recorder and now are doing beta test of a microammeter that you can plug in line to record flight profiles as well as static current draw. Watch the web site for announcements. Then you can check for yourself to see what throttle management really does. The new 4th gen HDR packs will be in soon and you can pop a 910 mah in your birdie that will live a long life at up to 18.2 amps max. Running it in your airplane at 9 amps will impose a load of just 10 C and the pack will deliver the full 910 mah capacity. Projected life under that condition will be several hundred cycles. That is what HDR is all about.

Happy holidays,
Fred
Agreed ,however the simple fact still remains that Hobby Shops ,especially ones with a repatation of dealing in electrics for many years should provide more details / clearer imformation when they are recomending a complete matched system. I could have thrown darts at there catalog and ended up with a matched system such as they recomemded.
http://www.hobby-lobby.com/shockflyer-super.htm

Charles
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Old Dec 18, 2004, 10:32 PM
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Charles, I looked at the Hobby Lobby ad you linked to, and agree that the ad by itself is misleading. To say that you can fly an aerobatic aircraft for 15 minutes on LiPos has no other meaning than a 4C average current draw. To recommend a 340mAh LiPo pack implies that you can fly the aircraft with the recommended LiPo for 15 minutes at an average 4C draw or 1.36A. This ad copy does not represent one of Hobby Lobby's brighter moments.

While that ad copy could easily mislead an e-flight neophyte, it's also pretty simple to shoot holes in if you have readily available basic electrical knowledge. So I have to say in this case that it's partially a case of a vendor not doing a good job of explaining and partially a case of a consumer not doing a good job of researching. It all goes back to the concept that e-flight is really not plug and play. It really does require some homework and fundamental understanding of how eletric motors and battery packs interact. Otherwise, you learn the hard way by blowing things up and burning things down.

By the way, the vendors who don't go out of their way to provide good, accurate data for their customers to make informed purchase decisions are going to lose those customers on future purchases because customers are not amused to learn that the ad copy that caused them to buy a product is deceptive. So, even though the customer may be somewhat at fault for not doing his homework, the customer is always right, and the vendor will be the ultimate loser.
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Old Dec 18, 2004, 10:58 PM
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FWIW,
with the 2208/34, APC 10x4.7 and 2S Kokam 340, I typically get about 7:15 flights in my Super Star Shock Flyer.
Full throttle is about 6.5 to 7 amps, but you rarely use full throttle for more than 10 or 15 seconds.
In level flight at full throttle the wings will flutter, and 10 or 15 seconds straight up and it is pretty high.
Hover is about 3 amps.
julianb, is the jeti controller one of the lithium-ready ones ? If not, perhaps you have been drawing the voltage down too low.
Pat MacKenzie
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Old Dec 18, 2004, 11:13 PM
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Pat, how did you measure the "6.5 to 7 amps?" It's interesting that this is lower than The Great Electric Motor Test numbers with the 10x4.7 prop, and even lower than Charles got with a 10x3.8 prop. A 7:15 flight on 340mAh cells suggests an average current draw of ~8C or 2.72A. So, obviously as you say, only a tiny fraction of the flight is using full throttle.

Good point on the controller possibly not being lithium-ready. Running the voltage too low is as proven a way to kill cells as trying to draw too much current out of them.
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Old Dec 19, 2004, 12:21 AM
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Measured with a Wattmeter, but it was last spring. I will do a new measurement to confirm it. I suspect the reason it is lower than the "GEMT"
results due to the voltage being lower than the 7.0 volts they used.

Pat MacKenzie
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Old Dec 19, 2004, 12:57 AM
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Just took some new measurements:

2S Kokam 340 6.8 amps 5.6 volts
This matches my recollection from the spring, right at 20C. The voltage should rise as the cells warm up, but with the Kokam pack I had the cut off set for 5 volts.
Even though I usually fly to cut off, the resting voltage is usually well over 7 volts.

2S TP480 8.4 amps 6.5 volts
New cells. Much higher voltage than the Kokam. I won't be able to try these until the end of January and that will be indoors. Very little opportunity for full power indoors. (TP recommends a 6 volt cut off, so I may have trouble switching between the Kokam and the TP)

2S TP1320 9.4 amps 7 volts
This is very close to the "GEMT" data point. With this pack the plane flys for more than 20 minutes, but is more sluggish. Vertical is better due to the higher voltage. Hovering requires more throttle.
Pat MacKenzie
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Last edited by pmackenzie; Dec 19, 2004 at 12:59 AM.
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