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Sep 13, 2003, 01:40 AM
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Mike-A's Avatar

reply to buzzsaw46


Yes, all cells are hazzardous, escpecialy when you get up in the higher capacity cells. But I have learned from this post as well as my own experience that some cells are more hazzardous than others.

It seems the combination of the highly flamable electrolite fluid combined with the "food grade" outer caseing -(lacking a nice hard containment can) this causes the pack to burst open quickly up to a 3 inch nozzel of fire verses just an end cap - plus long floppy solder leads that can often be poorly insulated by the user plus the fact that "saftey circuts" are actualy built into some -very few- cells, because they actualy prevent fire -this leads me to some conclusions.

All battery technology has fire concern, but lithium polymer has some of the greatest cause for concern. within less than a few seconds a shorted pack can burst into flame and because of this I will treat it with more respect than usual.

I liken it to my work. At times I go into countries in civil war i.e. D.R.Congo. I must allways be carefull, but in some areas I must be extreemly careful.

I respect your concern for Nicads -they too are hazzardous Please don't take me the wrong way.
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Sep 13, 2003, 04:58 AM
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buzzsaw 46's Avatar
Mike,

You are absolutly correct Lithium is more volitlie than the chemistrys most of us are used to! The reports of Li-Po's spewing fire is deffinatly sobering

And I still want someone who knows just what kind of damage it takes for a cell to become so unstable that it can ignite after an impact event, Is it the tabs are shorting or is it an internal short in the cell itself that leads to these type of fires??

Untill we know the exact nature of the failure there is no way to totally safe guard agaisnt it. lets face it eventualy you have to open the fire proof case and what if the cell decides that is the perfect time to spaw its hell fire right in your face!!!!

We need to know what kind of damage to be on the lookout for if these cells can in fact be damaged in such a way that they short internally. If it is a case of the tabs shorting than manufacturers need to build packs that allow for the inspection of the tabs, or insulate them extremely well. Myself I prefer them to be accessable to allow for ballance checks to help minimize the possiblity of an overcharge situation.
Sep 13, 2003, 06:45 AM
Registered User
"Myself I prefer them to be accessable to allow for ballance checks to help minimize the possiblity of an overcharge situation."

Me, too.

- RD
Sep 13, 2003, 08:58 AM
Registered User
Buzz, you nailed it. With all of the experiences and opinions posted about LiPo failures, I don't believe we have yet heard the definitive explanation. We just need to keep asking questions and hoping that cell engineers and other industry experts will show up here and answer the questions. I know a lot more about lithium cells than I did a year ago. But I'm far from comfortable that I fully understand all the failure modes that have been described to date.
Sep 13, 2003, 09:31 AM
Registered User
Quote:
Originally posted by Dave Hederich
Your definition of lithium polymer is a separate issue and highly controversial. It appears to be a minority opinion accepted by only a few people in the business. Since you've elected to raise the issue here, I'm sure that it will be hotly debated. After all, you are essentially accusing Kokam, FMA Direct, Thunder Power, E-Tec and a host of others of false advertising when you say that their cells are not true Li-polys. So I hope you aren't too sensitive about being questioned on the definition you've elected to promote.
I appreciate Dave pointing this out to me, as I'm not into hot debates. I only want to support the hobby with information I hope will provide food for thought.

The information regarding Lithium-ion Polymer vs. Lithium Polymer was posted so we as hobbies understand the technical differences between the two; this terminology is accepted in the battery industry. Bottom line is that we are using is Lithium-ion Polymer batteries and we will continue to call them Lithium Polymer and thatís ok, they work extremely well for our applications. We will market our line of Polymers as Lithium Polymer batteries as that is what is accepted. Itís not false advertising as the terminology is generally accepted.

The real issue is safety!!!!!!!!!!

The two most important safety issues are shorting (discharging) and overcharging (voltage).

Information regarding solutions for the overcharging (voltage) is available, easy fix. As far as the shorting, no one would intentionally short a battery so we donít have to discuss that. Although you should verify that the terminals are well protected.

As we know the Lithium Polymer is a very light battery and the reason is the casing. The aluminum container is very thin and is acceptable to impact and deformation. As long as the pack is protected and doesnít come into hard contact with anything the possibility of a problem is minimal. Most of your cell phones use Lithium Polymer batteries and we donít have problems with them, but they are protected inside of a hard plastic case. The cell/packs we are using can be damage if in a crash, if the pack takes a hit especially if it is visible then much caution must be taken. If the internal parts are compromised then a slow burn can start and intensifies with time, we all have read about the end result. I have on several occasions when testing done this. What happens is that the internal cell power generating material + and - is separated by a thin textile. When this separator is damaged a small short starts and so it goes. Bending will also compromise the materials in the cells. Take a cracker and bend it slowly, you will see the fibers at some point starting to separate. This is an extreme example but good for visual understanding of what happens inside a cell. The cells have relative good flexibility but there is a limit to everything. So, after a crash be aware and donít take any chances with that pack.

You guys have a good weekend Iím going flying.

Emory
Sep 13, 2003, 10:06 AM
Registered User
buzzsaw 46's Avatar
Emory,

That clairifys things for me to a certain point, But it brings up one more question. How long should a damaged pack be isolated before we can consider it relitively safe? I know that the exact length of time will vary depending on the sevarity of the dammage but it would be nice to have a ball park figure of how long a "slow burn" can take to intensify into a full out of control burn?


Thank you so much for providing this information.
Sep 13, 2003, 10:22 AM
Suspended Account
Quote:
Originally posted by buzzsaw 46
Emory,

That clairifys things for me to a certain point, But it brings up one more question. How long should a damaged pack be isolated before we can consider it relitively safe? I know that the exact length of time will vary depending on the sevarity of the dammage but it would be nice to have a ball park figure of how long a "slow burn" can take to intensify into a full out of control burn?
That answer may be impossible without the qualifier of "relitively safe". The damaged cell may be just a tiny bit away from a short and be a fire just waiting for the next minor shock to the pack.

Guys, I don't believe that one of the major suppliers of high power LiPo packs is still asking his customers to send the damaged packs back to him for repair/inspection. Sure hope they aern't sent air freight..........

Brad
Sep 13, 2003, 10:38 AM
Been There! Done That!
boomerace's Avatar
Brings up a good point! What is your libility if someone is injured by your cell since you have been informed of the danger by this forum and can't say "I didn't know!"
boomer
Sep 13, 2003, 10:49 AM
Registered User
buzzsaw 46's Avatar
Brad,

Thanks for pointing out the incorrect wording of my question and also for pointing out that any cell that as been through any type of impact may just be waiting for the next small shock to unleash it's fury

Boomer,

That is a very good point! I dont think I would feel safe sending a pack with visable damage through the mail system with out first doing the salt water soak. I can picture it now, The Feds pounding the door down to arest me for sending a mail bomb
Sep 13, 2003, 03:52 PM
Space Coast USA
hoppy's Avatar
I bought a "cash box", (small metal box with a latch), and use that to transport batteries to and from the flying field. Most stores that carry office supplies have an assortment of cash boxes.
hoppy
Sep 13, 2003, 05:32 PM
Flying Free
erashby's Avatar
Quote:
Originally posted by hoppy
I bought a "cash box", (small metal box with a latch), and use that to transport batteries to and from the flying field. Most stores that carry office supplies have an assortment of cash boxes.
hoppy
Be careful with the metal cash box not to let any batteries short out. I also store my batteries in a metal box-an ammo box.
Sep 13, 2003, 06:10 PM
Space Coast USA
hoppy's Avatar
Thanks for posting the precaution. I have a piece of self adhesive shelf covering tin the bottom. It has the handle on the top which makes it easy to carry without dumping everything around inside.
h
Sep 13, 2003, 06:22 PM
Registered User

Another safety issue


Another safety issue, at least according to Radical RC, is having a Li Po pack out of balance. In the instruction sheet shipped with their packs, Radical RC states:

"An out of balance pack will eventually catch fire."

Yet to the best of my knowledge, only Radical RC includes a center tap as a convenient means of measuring and charging individual cells. Are not the other providers of Li po packs doing a dis-service to users (and also creating a potential legal liability for themselves) by not providing a convenient means for users to guard against cell imbalance?

Maybe some of the other Li Po pack providers would like to comment on why they do not incorporate what would appear to be a very low cost safety feature in their packs. Or, alternatively, explain to me (and other readers of this thread) why it is really not needed.

BTW, I have no financial interest in Radical RC whatsoever.
Sep 13, 2003, 07:42 PM
Space Coast USA
hoppy's Avatar
They all recommend checking the balance before charging series packs but like you say, how?
hoppy
Sep 13, 2003, 09:08 PM
Registered User
Emory, you bring a lot to this forum and I hope that you will continue to post here and help us become more knowledgeable about Li-poly cells and safety. It's good for us to know that Li-poly cells can either be "wet" or "dry," so I really appreciate you pointing that out to us. The fact that the Energy Storage Association lists both wet and dry technologies under Li-poly with no distinction as to whether one type fits the category more than the other pretty much eliminates in my mind the issue about whether one type is more of a "true" Li-poly than the other.


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