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Old Nov 27, 2010, 09:49 AM
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Fully charged LiPo lost all power in few seconds in cold weather ?!?

I fully charged all my lipos last night. Checked capacity and voltage, everything was fine.

This morning I went out flying, it was a bit chilly 0 C (32F). Everything went fine, until the 3rd pack. Right after take-off the, the motor started pulsating ( low voltage cut-off protection ), I was lucky to land in one piece, same happend with the 4th battery ( but this time I check full throttle on the ground ) same thing happend. 5th and 6th was fine !

At home, once the batteries warmed up, I check voltage for the two that I had problem with. it was full. I put into the plane and went full throttle. Everything was fine, it ran it for a couple of minutes. no problem.

It looks like some lipos take cold weather pretty badly. I knew that cold slows down the chemical reaction, but this much? to loose all power in seconds?

Anyone has similar experience?
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Old Nov 27, 2010, 10:19 AM
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Haralson County GA. USA
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http://www.rcgroups.com/forums/showthread.php?t=1343928
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Old Nov 27, 2010, 10:34 AM
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Rugby, UK
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This is a well known phenomenon and is caused by the IR of the cell rising due to the low temperature, so much so that the voltage drop is sufficient to bring the voltage of a fully charged pack below the Esc cutout voltage.
If you look at the plots below you will see that from our normal flying temp (25 deg cent to 0deg) the IR virtually doubles.

Wayne
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Old Nov 27, 2010, 11:05 AM
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Nice graph Wayne!

Quick question - were these all lipolys? Reason I ask is that pack C has a dramatically different curve than packs A and B, denoting a dramatically different temperature coefficient.

It was my previous belief (with little supporting data) that internal resistance temperature coefficients of various lipolys were basically identical or at least enough so to be considered inconsequential. Your graph turns this notion on its head. To date, I have been performing my IR measurements at room temperature. Might have to rethink this...

Mark
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Old Nov 27, 2010, 11:33 AM
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Quote:
Originally Posted by mrforsyth View Post
Nice graph Wayne!

Quick question - were these all lipolys? Reason I ask is that pack C has a dramatically different curve than packs A and B, denoting a dramatically different temperature coefficient.

It was my previous belief (with little supporting data) that internal resistance temperature coefficients of various lipolys were basically identical or at least enough so to be considered inconsequential. Your graph turns this notion on its head. To date, I have been performing my IR measurements at room temperature. Might have to rethink this...

Mark
the dramatically difference in IR would explain why some of my Lipo's lastes 7 minutes, some 10 seconds, I guess
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Old Nov 27, 2010, 01:17 PM
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In cold weather I keep my lipos warm
I have an insulated bag. This contains a small bag of rice which I heat in the microwave - not too hot, just nice and warm to hold. The bag of rice goes into the bottom of the insulated bag and the lipos go on top. This way the lipos stay warm for a long time. It is also nice to quickly warm your freezing fingers while getting fresh packs out to put in the plane >:-)
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Old Nov 27, 2010, 05:39 PM
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Rugby, UK
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Quote:
Originally Posted by mrforsyth View Post
Nice graph Wayne!

Quick question - were these all lipolys? Reason I ask is that pack C has a dramatically different curve than packs A and B, denoting a dramatically different temperature coefficient.

It was my previous belief (with little supporting data) that internal resistance temperature coefficients of various lipolys were basically identical or at least enough so to be considered inconsequential. Your graph turns this notion on its head. To date, I have been performing my IR measurements at room temperature. Might have to rethink this...

Mark

Thanks Mark,
Yes they were all lipoys and it surprised me as well; I expected them to all run approx parallel as per packs A and B do, but just at different levels. I did them 2 yrs ago for a joint lecture with Bob Smith and have just looked up my old notes to see what the packs were.
'A' was a Loong Max 3S2250 25C
'B' was an Electrolite 3S2200 22C
'C' was a RC Modelpower 3S2200 20C

I have a chart showing the initial dissipation within each pack at 20C which is unfortunately not in a format I can post but I will try to convert it tomorrow. It shows that the L.Max pack at 35deg dissipates 19W whilst at the other extreme the RC Modelpower at 5deg dissipates 174W!! and drops 4V which answers the original question in the thread.

Wayne
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Old Nov 28, 2010, 06:08 AM
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Rugby, UK
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Here is the dissipation chart for three different lipos.
If you know the ESR of any pack you can easily calculate the initial losses which makes a mockery of many "C" rating claims.
I have also posted a comparison between a 3S lipo (purple plot) and equivalent 7 cell Nimh pack (blue plot) Note that for a temp fall of 30deg.cent the Nimh ESR rises by 35% whilst the Lipo rises by 100%
As a direct answer to the original question, I have dug out a plot of three 10C discharges on a lipo at different temperatures. The yellow plot is a classic low temp. discharge.

Wayne
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