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LiPo performance decrease?

Old 11-19-2004, 08:04 PM
  #1  
civic619
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Default LiPo performance decrease?

Hello,

I believe this will be my first post here on this board. I currently have a GWS Dragonfly set up with 2 HS-55 Servos, GWS electronics 3n1 board, reciver and stock motor, DD tail, helihobby skids, modded swashplate with coller, Helihobby wooden fixpitched main blades and (the problem area possibly) 2 LiPo packs. ( Electrifly 2cell 1200mah using a Dynamite Vision Ultra Peak charger that does lipo.

Ive been in the RC hobby for a good 10 years now, predominantly into nitro/electric cars and actively raced when time permited. This is the first time I have experienced the use of Lithium Polymer cells. Here is my problem:

I recently purchased 2 packs of LiPo cells. Electrifly 2S 1200MAH packs to be exact. This is now their 15th cycle and their performance has decreased, for example.

After a pack has been fully charged ~ 8.4V, it gives me a good 5 minutes of strong fly time, then, after that point you can see the heli loose its punch. It can only hover approximately 6inches off the ground at full throtle. At this point, I measured voltage, which is around 7.3Volt. I let her rest and at that time, the voltage increases back up to 7.8 which then stabalizes. I give it another go and she flys strong for anothger 45 seconds. From this point on it repeats the process but doesnt get any better.

This characteristic applies to both cells except one of the cells performes slightly better than the other. I havent measured the temps yet on the main and tail motor but im sure they arent overheating.

Anybody have any clues as to what is going on here? Could my cells be bad on teh 15th cycle? I am still using the stock gear ratio if that helps any.

Thanks for reading.
Old 11-22-2004, 05:05 PM
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tnd2000
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Default RE: LiPo performance decrease?

You should measure the voltage of the individual cells in the pack. Yes you will probably have to break the plastic seal/tape that bounds the cells together (be sure it's only this tape. DO NOT PUNCTURE the battery). From what you're describing sounds like your cells are unevenly charged. This leads to a vicious cycle and will quickly destroy the battery. Try charging each cell individually and hope that fixes the problem.
Old 11-24-2004, 10:42 PM
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im_a_rcav8r
 
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Default RE: LiPo performance decrease?

Before you go messing with your LiPo battery, check with your local hobby shop! It may be a charger problem. Its funny that 2 batterys would go bad the same way about the same time.

If you do want to take you batterys apart, let me warn you. A LIPO IS NOT LIKE A NICAD!!! If you get air into the LiPo cell (by nicking the battery when taking the plastic off) you will see an impressive fireworks show. I saw my LiPo go up in flames. It scared the Heck out of me! Air and LiPo makes FIRE!!! Take a look at this link and you will see what a LiPo will, not can but WILL do when it fails!

[link=http://www.rotory.com/ubbthreads/showflat.php?Cat=&Board=UBB2&Number=38655&page=1&view=collapsed&sb=&o=&fpart=1&vc=1]http://www.rotory.com/ubbthreads/showflat.php?Cat=&Board=UBB2&Number=38655&page=1&view=collapsed&sb=&o=&fpart=1&vc=1[/link]

Or just do a serch for LiPo fire on Yahoo and you will see lots of other links! Some have video...

Please be quite carefull with your LiPo. Charging outside or in a fireplace or other container built to withstand 1700 deg. is a wise idea.
Old 11-24-2004, 11:13 PM
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tnd2000
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Default RE: LiPo performance decrease?

Just to be clear, I'm not suggesting taking the lipos apart but breaking away the tape that bounds the cells into a pack so that individual cells can be accessed.
It's actually a common problem with cheap batteries where the cells hold different charges when bought. When you charge a cell pack, you are always over charging some cells and/or under charging others. This is why some brands are so expensive. They take batch of 1000 cells and test charge and drain them and selects cells with similar capacity, charge/discharge rate and make them into packs.

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