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Old Aug 12, 2004, 07:34 PM
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Dan Baldwin's Avatar
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no, it didn't exactly take me the whole day to do the test. Actually I never would have worried about it if I hadn't seen such a baloney samwitch number posted. It was probably the left over coffee in the bottom of the cup that threw the measurement off Maybe we should start our own version of mythbusters.

Dan
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Old Aug 12, 2004, 07:37 PM
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Lehi, Utah, United States
Joined Dec 2001
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Apparently there are some who do not understand some fundamentals of electric conduction and ionization.

Trying to measure resistance of salt water with a meter is not going to produce the same effect as submerging a lipo cell. Ionization of salt water changes with the applied field -- along with the effective resistance. The higher the applied voltage, the more electrons will migrate, and the lower the effective resistance will be.

An every-day parallel is lightning. Huge fields ionize a path through the air that becomes more conductive than the air around it eventually allowing conduction of static electrons between clouds or between clouds an the earth. As with lightning, ionized paths through salt water are rarely straight or singular. Hence, as was said earlier by someone who knows what he's talking about, measuring resistance of salt water is tricky. Only an average resistance per volt is truly valid.

Ohm meters use tiny voltages and currents to measure resistance. Trying to apply those measurements to other circumstances is not sound science.

Here's the basic textbook theory:

2.1.9 - Conductivity of salt water

One of the characteristics of water is its high dielectric constant. When a soluble salt such as NaCl is placed in water, the molecules of the liquid immediately act to decrease the electric forces between the ions of the crystal which, when combined with their natural heat motion, causes electrolytic dissociation. When a voltage is applied across electrodes submerged in the solution, Na+ is attracted to the negative plate, or cathode, while Cl- is attracted to the positive, or anode. Since they are free to migrate, they will do so and will achieve a steady ionic velocity which is proportional to the field.
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Old Aug 12, 2004, 07:55 PM
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Actually my meter (like most DVMs) doesn't use a tiny voltage on the ohms scale. The open circuit voltage accross my leads is 2.5 volts, or over half the voltage of a fully charged LiPo. I think you're right though. There are some who do not understand the fundamentals of electric conduction and ionization.
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Old Aug 12, 2004, 08:15 PM
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Walled Lake, MI, USA
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Everyone knows the biggest baloney dispensers are the ones who post under multiple identities, right, "kit?" For instance, it's understandable why someone who had past run-ins with RC Groups management over failure to show respect for fellow posters while posting under a more commonly recognized handle might think it clever to hide behind a different identity to get away with more disrespectful trash talk.

Problem is, those who continually underestimate the intelligence of others while overestimating their own cleverness eventually fall victim to their own bad judgment.
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Old Aug 12, 2004, 08:41 PM
kit
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It would be fairly easy to measure the actual resistance that a lipo sees when being discharged in salt water. Just cut a couple of pieces of kitchen aluminum foil about 1/4" wide by 1/2" long. Glue them to a piece of plastic about an inch apart with some electrical connections to them. Connect a lipo cell to the two aluminum tabs through an ammeter. Dip the two tabs into the salt water and when the bubbling starts, read the amps and measure the voltage of the lipo. As I recall, R still equals E/I.

Btw, the area of the lipo tabs in contact with the salt water will only be 0.125 sq. inches which is quite a bit less than 3 sq. inches and I believe that would make the resistance value even higher. As my old prof used to say....I'll leave that as an exercise for you to do if you're at all interested in finding the answer to the problem.
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Old Aug 12, 2004, 08:53 PM
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If I get some time tomorrow, I'll do some more real world testing of current flow through salt water.

I guess that since I'm a green newby that's wet behind the ears, I should try harder to not ruffle any feathers. I will try harder in the future.

Dan
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Old Aug 12, 2004, 09:03 PM
...still seeking PyrE
Long Island's North Shore
Joined Aug 2003
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To answer a few more questions and perhaps give some of you more insight into the "Lipo question":
The pack had ballooned to perhaps twice its normal size - from the surface of the outermost cells, and the yellow shrink had cracked slightly in spots - while still in the tray, on the workbench.
No, I was frankly afraid to touch the pack before the gloves went on, so the question of pack temperature will not be answered here, but remember that the pack felt very slightly above ambient some 5-minutes before.
I carried the pack outside by the micro Deans plug, and am still not sure how I was able to grab hold of that plug with those gloves.
It was dark outside and the flood lights and front door light did not illuminate very well that far down the drive. But after the flames/sparks died down, although heavy gray smoke still rose from the container, I believe that the water was bubbling, but am not certain whether this was very hot water or some result of the chemicals just released. There may have been a sound as well, but certainly not sure...then or now.

The next morning in the daylight, the water looked black from the doorway but closer inspection showed that the greatest mass of the black material was accumulated at the bottom of the container. The container above the water line was spotted with black, which ran over the top and streaked down the outside.

After returning home that evening and taking the pack out of the water the 2- "exit wounds" were at last seen. There was a rather gaping - about .75 to 1.00 inch in length - opening in the side of the center cell and a similar opening in the outer surface of one of the outermost cells. These openings were black inside, shaped like lips, and clearly looked like they had been "blown out" from the inside; only the cell without an opening was quite a bit more swollen than the other outer cell, and the swollen area was relatively hard and difficult to compress to any degree. The rest of the pack was less swollen, but was equally "hard" and not easily compressed.

As much as I wanted to try and puncture the most expanded area, I was hesitant to do that - more out of fear of the result, notwithstanding the fact that the pack had burned and been in salt water for almost 24-hours. I then carefully separated the wires, cut each gingerly - those micros are precious, and put the remaining "mess" in the trash.

Some of the "darker side" of this experience is that I now have a compulsion to each evening, first: sniff the air - that sickingly sweet odor will not soon be forgotten, then to open each Sentry storage box (2) and look at the packs; my therapist is going to love this one.

Hope some of this will be helpful.

Stew
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Old Aug 12, 2004, 09:04 PM
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Waycross, Ga.
Joined Jun 2003
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Quote:
Originally Posted by Darren Hintze
I've been dealing with Lipos for a long time now...

Having had Nitromethane model fuel in my garage, I sleep much better knowing there are just lipos out there now.

Darren
Darren, I sleep better knowing I only have Nitromethane and NiMh batteries in my garage !

Tracy
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Old Aug 12, 2004, 09:06 PM
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[QUOTE=kit]Looking forward to hearing hoppy's WAG explanation of your resistance measurement Dan. This should be good! Well,......maybe not.[/QUOTE

I don't think you are going to like the answer kid -

Sea water is about 18 ohms/cm - Read it and weep kit - http://www.aps.uoguelph.ca/~aquacent...s/electro.html

The great salt lake calulates out to about 6ohm/cm

No hot air, just facts-

You have been checked and mated - LOL
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Old Aug 12, 2004, 09:33 PM
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If the University of Guelph says it, it must be true
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Old Aug 12, 2004, 09:40 PM
Just an average RC'er
Jim McPherson's Avatar
Laurel, Maryland USA
Joined Aug 2000
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That will be quite enough of the ad hom attacks. From EVERYBODY. Clean it up and stay on topic or this thread will be as closed as kits mouth should be.

TJCooper, most of us here know who you are and look forward to hearing your results on the underwater test.

-Jim
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Old Aug 12, 2004, 10:26 PM
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Los Altos, California
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This may not be totally relevant to this thread, but I recently did a destructive test on a 3S3P (6000mah) Lipoly battery.
And I felt that this might be of some relevant side interest, in light of the discussion at hand.

I will include a link to a video of the test..
Please note from the video that there were multiple ignitions of various portions of the battery over time...
Therefore, Don't just assume that the party is over when the first set of flames subside.....

The battery had actually been in a crash and was somewhat physically damaged. However there were no visible punctures or dents in the battery, and it was able to take a full charge prior to the test.

We wanted to determine how easy it might be to ignite a pack with extreme overvoltage during charging.
It turned out to be much more difficult than we thought.

So the test was set up to start the charge at 8S with a 3S pack.
The results will surprise you.. We eventually increased the charge voltage to the equivilent of an 11S charge rate.

The battery was placed in a high-fired ceramic pan with high walls.
A bag of sand was placed on the bottom of the pan, then the battery was placed in the pan and finally another ziplock bag with almost 3 inches of very fine grain sand was placed on top of the battery.

Here is a video of the test and the results.
(My wife is a potter and has been making these "battery bunkers" for a few of the local flyers).

Note that this is a Quicktime movie and will require that you have quicktime installed on your computer.

http://harley.rchomepage.com/ThunderPowerTest.mov

(BTW, I will be out of town for the next 2 days and may not be able to respond to any questions, until I get back)

Regards,
Harley
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Last edited by Harley; Aug 12, 2004 at 10:28 PM.
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Old Aug 12, 2004, 10:29 PM
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hoppy's Avatar
Space Coast
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Quote:
Originally Posted by Dan Baldwin
If the University of Guelph says it, it must be true
I'll agree with you on that -

http://www.aucc.ca/can_uni/our_unive.../guelph_e.html

hppy
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Old Aug 12, 2004, 11:31 PM
kit
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USA
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Off to do some fish electrocuting. Thanks for the link.

Those must be some different kinda ohms, like ocean ohms or lightning ohms or something.
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Last edited by kit; Aug 12, 2004 at 11:33 PM.
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Old Aug 13, 2004, 04:50 AM
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Joined Jan 2002
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Quote:
Originally Posted by tjcooper
... I have found that the metal tubes around the cells can really help control the fireball when a pack goes thermal. I hope many of you will consider using this method to transport and charge your packs. The firesafe is great for a few packs, but I can assure you that if one pack goes, they will all go.....one after the other. More than 5 packs in a firesafe could possibly cause the fireball to leak out of the safe. Only testing will tell for sure...
Ted

Your tube system requires fabrication which some folks might like to avoid and, as I understand the fabrication, the tubes are connected together. If used for transporting, the entire system would need to be evacuated from the vehicle if a pack burns in one of the tubes.

An alternative would be to use multiple Brinks Boxes, one for each pack, (They are cheap, Ca. $10 per), or surplus military amunition boxes (even cheaper).

- RD
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