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Old Sep 29, 2004, 07:33 AM
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very-short's Avatar
Lower Hutt , New Zealand
Joined Mar 2003
62 Posts
*** Lithium polymer cell for my Ultrafly ***

Hi all ,
I have a question as to what lithium polymer cells I can use on my ultrafly cessna 182 its spec say ....

ESC rated @ 15amps
recommended packs for the plane are ....
8C 9.6v 800-1200NImh
500-700 Nicd

I want to use polys for much longer flight times , My ESC is a sky victory 210 rated at 50amps is this ok to use with polys ??

Thanks for your help .
Cheers
Andrew
New Zealand
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Old Sep 30, 2004, 07:03 PM
Motor Maniac
Orange County, CA
Joined Feb 2002
2,875 Posts
You can use just about any ESC with LiPoly with a few cautions thrown in. What will kill LiPoly batteries in use (as opposed to while charging) is letting the voltage get too low. Some ESC's have the LVC at voltages that are pretty good for LiPo, like 5.5V or so, but others may have the LVC set lower, which is not good if you are planning on running the pack down to the LVC in flight.

What I do, even on my ESC's that have LiPol friendly LVC, is to time the flights so I don't discharge the pack too far. This means I will always fly a shorter time than the pack will allow, but it keeps the battery happy. One way to figure how long you can fly is to do a static amp measurement, then calculate how long it will take to empty the pack at that rate and put in a safety factor of 10% or so. A better way is to fly for 5 minutes (assuming you won't discharge the pack too far in that time), then measure how much charge the pack takes and figure your in-flight discharge rate from that. Still best to add a safety factor in this case as well.

Since the recommended packs are 9.6V you have to choose whether to use 2S or 3S LiPoly, nominally 7.4V and 11.1V respectively. You may have to regear or reprop as needed to keep the pack and motor within recommended operating ranges.
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Old Sep 30, 2004, 07:36 PM
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Joined Aug 2004
80 Posts
In the past I have had very good luck replacing an 8 cell 800 mah nimh pack with a 1500 mah 2-cell lipoly. ( I used a kokam 1500 2s1p) A 1500 to 2000 mah 2 cell may be appropriate for you.

The voltage is nominally lower on the lipoly, but because of the high impedance of the nimh, the voltage at full throttle from the lipoly was, in my case, actually higher and I got more power, plus longer flight, plus less weight. Doesn't always work out that way.

It would be really good if you could see what voltage your ESC will cut out at. If it will cut out above 5 volts, it will provide some protections for your lipoly if you use 2 cell. This is why I only use 2 cell.

The power comparison depences a lot on what batteries you are replacing. If you are comparing to nicad you will definitely get less voltage full throttle than the nicads. But even if the voltage with full throttle is a little lower, the decreased weight may make it fly better. The battery pack your replacing, based on the guidance from the manufacturer, is probably around 180 to 200 grams, ballpark. You could cut that in half with the LiPoly.

Since you are using brushed motor, you might want to be sure it can handle the voltage from a 3 cell pack before you try that. I've seen people burn up their motors after just a few flights when they switched to a 3-cell pack, even if they did prop down.

And make sure you have a pretty good idea of the current before you settle on the lipoly batteries. You have to be sure the lipoly's can handle the max current draw.

Something like a kokam 2000 mah 2 cell pack might be good, if somewhat expensive. It can handle 30 amps and weighs about 120 grams. An Apogee 1670 mah 2s1P can handle 22 amps max and only weighs about 76 grams. I personally am fond of apogee packs lately.

A little rambling, but hopefully helpful.

Dowd
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Last edited by dowd; Sep 30, 2004 at 07:40 PM.
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Old Sep 30, 2004, 09:14 PM
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very-short's Avatar
Lower Hutt , New Zealand
Joined Mar 2003
62 Posts
Hi Guys ,
Thanks for the info .... the min amp rating was 15 and peak is 22 so I think I should be fine with what you recommended DOWD " kokam 2000 mah 2 cell pack " My ESC has onboard BEC so that is a little low to use as a guid for my cell voltage I might run the motor { when I get the right packs } for a 5 min stint and measure the batt voltage and calculate from there what sort of flight times I might be looking at .I really wanted to know .... if my ESC would handle Li pos and what C rating I will need .Thanks guys you have answered my questions !
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Old Oct 01, 2004, 06:51 PM
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<deleted because I accidentally posted twice>
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Last edited by dowd; Oct 01, 2004 at 06:57 PM.
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Old Oct 01, 2004, 06:56 PM
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My intent was really to make sure you look at packs that are designed for your peak amps.
Like I mentioned those Kokams are kind of expensive. I would recommend looking at some other packs too. For example I was just looking at the apogee packs at www.pfmdistribution.com and they have a 2080 mah 2s1p that weighs 97 grams and can handle 29 amp peak. I think that pack is about $50. I imagine other people might recommend different ones that would also be suitable.

Dowd
(trying to be a little more brief this time)
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