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Old Apr 28, 2003, 04:48 PM
WAA-08 THANK FRANK!
JimNM's Avatar
Las Cruces, New Mexico, United States
Joined Jun 2002
7,348 Posts
What happens when you push a Nimh too hard?

I was loaned a battery pack by a local member, and I think I have mishandled it's care and feeding. RLoose - I will replace your batteries!

I have been trying out an homebrew 8 cell pack of the Ray-O-Vac AA Nimh, and I got it a little hot - Ok, a lot hot. I was bench running the motor at about 60% power and I got a little distracted. When I got back to the workroom, the pack was really warm - the plastic covering on some cells shows a small split and discoloration (like the cell expanded?) I let the pack cool overnight and then charged it up - runs like a dream on a smaller prop. MotoCalc shows ~ 16A static. I ran it today for 30-45 second runs and had a great flight - pack hot, but not so hot that I could not hold it in my hand with fingers closed around it.

Will the pack spontaneously detonate, melt down or just lose it's electrical continuity if I run it too hot or run it too long too hot? I would like to use this kind of pack due to the weight savings - but it's not worth it if I am limited to 30 second runs or risk losing the glider.

"Cheap" is my middle name.

JimNM
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Old Apr 28, 2003, 09:49 PM
RIP Ric
Andy W's Avatar
Marietta, GA
Joined Jun 1999
43,312 Posts
YIKES! AA NiMH are good for 10A, perhaps 12-14A peak for SHORT PERIODS! In flight you're probably OK, but NO MORE BENCH TESTING! You can drop current 10%, perhaps as much as 20 or 30% or so (depending on the prop, kind of model, etc.) in flight compared to static figures.
The shrink shrank and split, the cell did not grow. The case is too strong for that. It might explode, however!
..a
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Old Apr 28, 2003, 10:02 PM
tic
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New Cumberland, PA. US
Joined Dec 2000
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usually, they will vent... you can hear em pop
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Old Apr 28, 2003, 10:51 PM
Space Coast USA
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Space Coast
Joined Oct 2000
21,169 Posts
The long term result of abuse is a much decreased ability to supply current compared to a new cell. I have a bunch of 700mah AAA cells which were abused (HOT after flight) and after a couple of months I could only obtain a max 4A from them as compared to 7A when they were new. They work fine at 4A, but that's all they will supply (and don't do that very well till they warm up).
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Old Apr 28, 2003, 11:46 PM
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Re: What really happens.

!
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Old Apr 29, 2003, 12:24 AM
WAA-08 THANK FRANK!
JimNM's Avatar
Las Cruces, New Mexico, United States
Joined Jun 2002
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Well, I'm only about 150 miles from the TRINITY test site... so I might as well keep up with the Jones'....

I did not observe any leakage or venting. I had to cut the pack open to fix a connector that I broke by squeazing the pack - so far so good. I doubled the air intake area today, and I will see if that will keep the temps down.

JimNM (I may have change that to Boom-Boom )
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Old Apr 29, 2003, 10:57 AM
Space Coast USA
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Space Coast
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The throttle will keep the amps down better than anything

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Old Apr 29, 2003, 01:55 PM
WAA-08 THANK FRANK!
JimNM's Avatar
Las Cruces, New Mexico, United States
Joined Jun 2002
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Well, I got to fly today before work (about the only thing I like about the 2p-12a shift) I had the 8 cell Nihm pack warm, the 12x6 on the Endo 3.33 and a great deal of smooth air to fly in. The homebrew pack will work, but it does not have the punch of the 6 cell Nicad from Radio Shack. For those days where the air is SUPER smooth, the Nihm will be an advantage due to the lower AUW. The Nicad is just a little heavier, and with the greater discharge ability, has more "push" to get the plane up and through any rough air. The Nicad will turn a 13x7, where the Nihm just can't quite do it. I knew I was pushing the limits - so I'll keep it anyway as a backup. Rloose gets new cells, adn we both win.


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Old Apr 30, 2003, 10:21 AM
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Boise
Joined Sep 2002
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Sometimes, this happens...

even when you don't push them that hard... These are Kan cells in a S400 DD application.
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Old Apr 30, 2003, 01:30 PM
WAA-08 THANK FRANK!
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Las Cruces, New Mexico, United States
Joined Jun 2002
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Wow! That is pretty ugly. Was there any residue from the vent/rupture? If there is, how do you clean it out?

Did you still have any current - or did the cells fail in flight and leave you high and dry?

Thanks for playing
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Old Apr 30, 2003, 01:40 PM
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Boise
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No residue or powder at all...

the one side of the pack simply got too hot somehow (probably a short) and melted the cell wrap and the shrinkwrap. Fortunately, I was facing the wind and was able to land her normally with full control available (but with hardly any juice to the motor)....
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Old Apr 30, 2003, 08:31 PM
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wyoming, mi usa
Joined Dec 2000
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Mine exploded!

I nosed in a plane and had a big coin ahead of the battery for balance. I believe the cion helped to dent the battery into a short.

I saw the shrickwrap splitting and ripped out the pack. It was hot and blistered my thumb a bit. As i was holding it, I then saw it venting steam, and all of a sudden, KA-BLAM!

The top shot off and I pitch the pack and did a finger count. Phew. All there.

5 hours later and I still ahve hearing loss in one ear.

I have my all my fingers and both eyes. I was dang lucky.

rt
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Old May 02, 2003, 12:12 AM
WAA-08 THANK FRANK!
JimNM's Avatar
Las Cruces, New Mexico, United States
Joined Jun 2002
7,348 Posts
GULP! Ok, Ok - enough of the horror stories. pushing the AA's that hard is too risky. I had to read your last post twice... I saw that you had "all three" fingers the first time :eek

I will treat these little cells with a gentler hand, lest I lose parts of mine.

JimNM
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Old Nov 26, 2005, 12:41 AM
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Quote:
Originally Posted by Aslander
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actuly thats my computer them it did spell check on one of my posts
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Old Nov 26, 2005, 08:36 AM
turn, turn, turn.
Athol, Massachusetts
Joined Oct 2005
10,495 Posts
After looking at the exploded cells, Lipos seem more user friendly. I'd much rather have a meltdown than an explosion.... Thanks for the info and the rather "secret" warning...Never seen any hype about exploding cells, just a lot of hype about lipos.
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