Much thanks to DNA for putting up his pictures of Li-poly venting and catching fire:
http://www.rcgroups.com/forums/showt...5&pagenumber=1
and video at:
http://www.rcstuff.us/battery/lipo/lipo_DNA.html

This thread is a little different in that the cells "went bad" the night before and I tested how far they needed to be pushed to melt down.

I also have big safety concerns because what caused this problem to start was a Triton charger that has been used for 9 months now and given no problems up until this time.

I conducted a rather scientific study in making the Li-poly explode. I used a precision power supply to give the .72 C charge which caused the explosion, I used both an optical IR temperature gun and a thermocouple to measure temperatures. We had two different videos of the event so you can see it from different perspectives.

The cell was a CBP 2200maH made up as a 3S1P when the Triton was charging it the night before. A single cell which was badly "ballooned" was then put on the cement barbeque pit and pushed to its limits with less than 30 minutes of charge.

Please see the Li-poly Code of Conduct
http://www.rcgroups.com/forums/showt...hreadid=149283
for many good safety practices when handling Li-poly.

Here is the venting picture:
Ted Cooper

Sorry for the long document but I wanted to DETAIL all the steps that made the original pack go bad and then how we tested the single cell that exploded:

Original cell pack problem:

The cell that was tested had previously swelled up night before. It was the third cycle of discharging the battery at a full power level to cause it to reach its maximum power potential. It was flown with a PJS550E motor on a P51 mustang as part of “cell conditioning”. The third such cell conditioning had been completed with no observed problems. I was recharging the battery on my L200-type homemade charger. I had reached the 10.2-volt stage when I switched to the Triton charger so I could finish the charging before midnight. I set the Triton manually to 1.8 amps at 11.4 volts for the 3S1P pack. I had looked at, and touched, the pack prior to switching to the Triton and there were no problems. I continued to work in the garage on another plane at a different workbench while the charging took place.

After 22 minutes on the Triton I checked the pack. It was at .7 amps (the largest rate it had reached) and the two upper cells were swollen to ten times their normal size. I grabbed an IR heat sensor gun and measured that the top pack (the hottest one) was at 104 deg F. The pack was located inside the nose of the P51 and could only be observed through the wing saddle. I put the plane outside on the cement to cool, and then surgically had to cut apart the three cells so I could remove them through the fuselage.

Baylands setup:

The next day at Baylands I took the most swollen cell and performed a destructive charging test. I used a HP precision Lab supply to drive 1.58 amps into the cell until it exploded. I placed the cell on a foam wing bed to simulate what would happen inside a plane. I connected a thermocouple to the bottom of the cell to get continuous temperature readings. The optical and thermocouple agreed that the cell started to 69 deg F prior to the charge which started at 8:55 on Sunday morning. At 9:21 am the cell started smoking as the surface temperature reached 101 deg C (214 deg F). Within 20 seconds the cell ruptured at both ends and shot out thick white smoke. A few seconds later the smoke burst violently into flames that lasted for approximately 3 seconds. The foam base caught fire and the pack temperature reached 219 deg C (426 deg F). The pack threw 1/32” sized pieces of metal debris about 5 feet from the pack. These small pieces melted small indentations in the handle of the IR heat gun which was 4 feet away.

It should be noted that the cell immediately started to gain in temperature as soon as the 1.58 amp charging started on Sunday. The cell reached 150 deg F within 20 minutes of the commencement of charging. Tiny amounts of smoke rose from the pack roughly 5 minutes before the pack vented the white smoke. An observant user would easily have known that something was wrong and been able to evacuate the area. However, once the smoke started rising from the cell, I doubt that removing the charge current would have prevented the pack from exploding. Thermal runaway probably occurs when the pack external temperature reaches 95 deg C (203 deg F).

Here is the fire picture:
Ted Cooper

There is no voltage data recorded for the "bad" cells so it is hard to figure what you are actually saying here. It appears (stress the appear) that two out of three cells were already driven to an overvolts condition on the Triton - what was the voltage of the third cell? Had it failed short or had the two "puffers" gone low capacity?

Once the cells have ballooned, you KNOW they have been damaged.

The outside test appears, from your text to have been an uncontrolled, forced current, test. What was the voltage limit at that time?

We all know (or should) that overvolts is dangerous on this chemistry. Put another way, the key to avoiding this sort of pyrotechnics is limiting cell voltage. I suspect that the volts were through the roof before the controlled explosion.

Jim Davey

I have worked with Mike and CheapBatteryPacks.com for quite some time now. He was concerned, and rightly so, that showing pictures of Li-poly explosions would cause the "Nay sayers" to proclaim that Li-poly is not safe enough to use. The old ones of the flying crowd remember NiCds that set airplanes on fire in their early days. Nothing has changed! The big problem is understanding what can cause the problems and how to take safety precautions to protect you and those around you.

DISCLOSURE: the 3S1P pack from Cheapbatterypacks.com was constructed by me with standard methods I have used on over 50 Li-polys that I have made this year. This is the first problem (not of my own making) that has occurred. This pack was NOT one of my experiments in "bending" a Li-poly or doing short-time over charging. All of those packs are still working just fine.

PROBLEM: in the 22 minutes that I had the 3S1P pack on the Triton, I watched the current readings approximately ever 5 minutes. The unit never got above .7 amps of charge and the voltage never got above 11.3 volts when I observed it. The fact that two of the three cells ballooned up means this was not a simple "cell gone bad". Something pushed the pack over the edge! I fear, but have no proof to back it up, that some voltage and/or current spikes happened during those 22 minutes of charge. I will run other tests on the Triton with other Li-polys with an oscilloscope and a recording voltmeter to see if it sometimes "goes crazy" and over charges.

Another thing that bothers me is that ballooning occurred at such a low temperature (under 104 deg F). Mike has tested many of his packs at 85 deg C with no problems. So why could two cells start to balloon up with so little heat being present?

LESSONs LEARNED:
1) would I stop using Li-poly? ABSOLUTELY NOT! I only fly my old Zagi on NiMh any more. Everything else flys on Li-poly. It is like going back to riding horses once you have driven a car. The Li-poly have too much going for them....and they get better every three months.

2) NEVER put a Li-poly in an exclosed frame where you cannot see it when you are charging it. It has to have airflow when it is being charged (not that it gets hot). But you have to be able to see what is happening to the battery.

3) my 2" tall CorningWare dish is going to be replaced by a 6-8" tall dish really soon. The shrapnel that the cell threw out convinced me that something more substancial is needed. However, I think a simple ceramic dish is more than enough to contain the momentary heat and frame front. I see no need for major steel containers and fire-proof vaults (but I could be proved wrong on this in the future). Also, DO NOT cover the top. The venting and fireball convinced me that a "top" to the container would just be one more flying object at the scene of the accident.

4) NEVER EVER charge without being in the same area. It took 22 minutes for the pack to balloon to the point were it completely filled the fuse of the P51 Mustang. It probably means that you should "come touch the pack" every 10 minutes. I doubt anything major would happen on a 10 minute time scale. But it also means I cannot go into the house and watch a TV program for 30 minutes. Every single commercial break is a trip to the charger.....at a minimum! Best to just stay there and work on your other planes or other tasks. The smell of the venting was very strong. It will alert you to problems.

5) If you see white smoke - of any quantity - get the plane and yourself out to a safe place and then keep at least 15 feet away. I have asked the Sony Li-poly people to give me some details on when a Li-poly goes "critical" such that removing the charge will not stop the runaway venting and possible explosion. I will pass along any info I obtain.

VIDEO:
the first video was by Jason of Sheldon's Hobby who was there (much thanks). It just covers that time of final venting to flames. It is 5+ Megabytes and is .mpg in format. This video shows the setup of equipment and me with safety equipment around the test.
http://mywebpages.comcast.net/tedcooper/LiExplosion.mpg

the second video is my ancient analog camcorder at three feet from the pack. The shaking video is from the old camera not working too well. You can clearly see the temperature meter (in CENTIGRADE) behind the pack. If you play with contrast and brightness, you can see the voltage and current on the HP supply. It was set for constant current at 1.58 amps. The "humor" during the test was mostly my way of keeping the other flyers from getting too close or being too nervous....much thanks to all the "helpers" who were taking cover behind the steel poles in the picnic area. Note the sound level....the explosion was about the level of a medium sized firecracker.

This video needs a newer class DIVX driver to view. Download it at www.divx.com

This video is 16.6 Megabytes and has been "de-interleaved".
http://mywebpages.comcast.net/tedcoo...losiion2_d.avi

Ted Cooper - sometimes known as Crazy Ted

Ted, first of all, thanks for sharing your experience. There are always lessons to be learned.

>> The pack was located inside the nose of the P51

Not that it mattered in this case. But it should be pointed out that charging a LiPo pack inside an aircraft is contrary to suggestions in Jim McPherson's Ultimate Guide to Lithium Polymer Batteries, which is based on the cumulative experiences of many LiPo users who have experienced catastrophic cell failures. The truth is that the cells could have vented in the aircraft while Ted wasn't looking, and caused quite a fire on that other workbench. I hope that everyone reading this thread takes note that this is a good example of why LiPo packs should be charged in a safe location, as recommended in the Guide.

EDIT: Ted, I see that you were composing your last message at the same time that I was composing mine, and that you also pointed out the recommendation not to charge a pack in the aircraft. I'm going to leave mine up as reinforcement.

teamdavey,
you got in there while I was posting everything so you probably did not see my notes at the end where I talked about voltage and current readings.

Night Before: The Triton was set MANUALLY for 1.8 amps at 11.4 volts for charging our 3S1P pack. My handmade charger had already taken the cold cells to roughly 10.4 voltages before I switched to the Triton. The cells were ambient temp. I tested the Triton charge for the first 5 minutes and it was still going through the ramp up with a max voltage around 11.2 volts to get .5 amps into the cells when I stopped watching. When I noticed the batteries in their ballooned state, the Triton was at .7 amps and the voltage was at something like 11.2 ( this is the driving voltage, not the resting voltage where Triton monitors the current state of the cell).

Baylands: the constant current was set to max (1.58 amps) the voltage was approx. 5.2 volts for the single cell -- and yes, we definitely were over-driving the cell from a voltage standpoint. That was our goal. To see how long at constant current it would take to either short out or explode the cell. I will have to look at the videos again, but I think the final voltage before explosion was like 6.5 volts????

The third cell from "the night before" appears to be fine and has a reasonable charge in it. Needless to say it will have to be destroyed just because we do not know what happened.

I hope this answers your question - but I did not completely understand what you were asking. Sorry.

Dave,
I ABSOLUTELY agree about the not burying a battery in the fuse. Mine was not really buried, you can see it through the motor mount and it is completely open on the wing side but the spar does not allow the battery to come out. Yes, no one should ever charge any battery INSIDE the plane. It is just setting you up for problems. I have learned my lesson.
Ted Cooper

Thanks, that was some of the data I was after. Given that voltage is the key it would have been nice to know what the individual cell voltages were on the Triton since that data probably contains the evidence for the first failure. That, however, is lost in the mists of time.

Jim Davey

I agree with Jim Davey that the key to the
mystery may well be within the third cell.
Do you still have it? If so, what is it's voltage
level? If significantly low it may well have led
to an over-voltage condition in the two ballooned cells.
Perhaps the answer, other than charging and storing the batteries
where they can blow up in relative safety, is to charge the cells in parallel
rather than in series and to use over-voltage
protection circuits.
-Dave

Perhaps the answer, other than charging and storing the batteries
where they can blow up in relative safety, is to charge the cells in parallel
rather than in series and to use over-voltage
protection circuits.
-Dave

AMEN! If we charge in parallel only, a dedicated 4.2V charger is all we need. Careful regulation of the output voltage with failsafe circuitry is relatively easy, compared to trying to anticipate all possible failure modes with selectable voltage series charging circuits.

- RD

Thanks for taking the time and effort to document a failure.
hoppy

Blue Sky and Blakeslee,

I did measure the 3rd cell in the pack. I don't remember the actual voltage but it was something like 3.9 volts. I will remeasure it again tonight along with the 2nd balloon cell which is still in tact. I remember measuring all three cells and commenting to myself that they were all in the same "ball park", but I will take the measurment again. The "exploded" cell was at 3.92 volts when I took it to Baylands on Sunday morning.

In the early 1990's, several NiCd fires in homes (actually in garages and cars) prompted pundits to say they should be charged in parallel to equalize the packs and find out who the "weak sisters" were. Except for 20-cell racer configurations, does anybody routinely take their NiCd or NiMh apart for parallel charging......anybody in the universe???? It just does not happen. Think about all the helicopter packs with 3S6P that are out there. The only breakdown I have seen is 3S3P sub packs that are charged separately.

A Li-poly charger costs around $100 if you get 1,2,3 cell and up to 1.5 amp variable charge rate. At .8 C (which is what I charge at), it would take 4 hours to charge a 3S1P pack with each cell being charged. No one is doing that! No one probably will. I created a 4 independent Li-poly charger system out of L200 chips and I am still not able to keep flying continuously on Saturday and Sunday (although I make a really good effort at it!).

I have over 40 Li-poly packs that I use right now. Most are 2S1P or 3S1P types (but 3S2P is also common). From time to time I take the tape off the connectors (I build almost all of my own packs), and measure the voltage imbalance. In the 9 months I have been doing this, the largest imbalance I have measured is .1 volts in a 3S1P pack (which is nearing the end of its lifetime anyway). A typical pack shows something like .02 volts imbalance between highest to lowest cell in the pack. I only wish the NiMh I have used were that matched! So from practical experience, I do not see cell imbalance as a big factor. However, I also discharge only at 3C to 5C with packs that are probably good for 5C to 10C in what is being advertised. I also "land early and often" to make sure that my packs are not discharged below 3.0 volts per cell under load.

How do I know when I reach the 3.0 volt level? When the flying gets "mushy", I land. When hovering becomes "iffy", I land. I have bench tested a number of motors with Li-poly and found that when 6.0 volts (for a 2S1P pack) is reached, that within two minutes after power off, the static voltage is 7.2 volts. I always charge immediately after landing (or try to). If I ever see the Triton or the L200 start at lower than 7.2 volts on the scale, I know I have been "a bad boy" and over discharged. I then adjust my level of "mushy-ness" for that pack so that the next time I fly, I land earlier. It has worked very well. I have not yet had a pack that I have made give me less than 150 cycles -- although I must admit that I only have two packs that are in that much flying time.

CONCLUSIONS: people are not going to dis-assemble their Li-poly packs to individual cells......it just will not happen. And charging multiple individual batteries in parallel on one charger is a "crash" just waiting to happen. Until Hobbico makes a 6 independent charger for under $100 with lots of little aligator clips, no one is going to do it. Serial charging up to 4 cells works, ....and works well. It is good practice to periodically (like every 15 recharges) get a voltmeter and measure the imbalance. But I have not seen anything to suggest it is a major problem if you do not push the batteries too hard. All of the Li-ion and Li-poly that Sony puts in computers and Walkmans charge in serials (although they do break down the cells sometimes so they are not in parallel). But they are very conservative in charging and discharging.

The bottom line is that serial charging is here to stay. I invite anyone with data on cell imbalance from practical experience to please tell their stories. It will be very useful to the community.

Again, the soapbox is available for the next user!

Ted Cooper

"I invite anyone with data on cell imbalance from practical experience to please tell their stories."

http://www.rcgroups.com/forums/showt...st#post1321519

- RD

Very nice piece, Ted.

I suspect that a lot of the surprise unbalance issues are down to deap discharge. If the cells are in balance at a high State of Charge they are liekly to stay that way other than through excess self discharge. Since modellers aren't leaving the batteries lying around much I don't expect that is an issue.

However, if you deap discharge, and the cells capacities arn't equal......

My own sample on Lipo is too small to be statistically significant but is very positive so far. On other chemistries - deap discharge is always iffy.

Jim Davey

Blakeslee,
I read your other thread and was quite amazed at the difference. Do you think there was anything unusual about the three cells you tested?

I have noticed with the E-tac and CBP cells that I have to "hard discharge" them for at least 3 cycles before they perform well. "Hard discharge" basically means putting them on the airplane and discharging them staticly in my garage (like the P51 I did in the garage). Maybe during those first three cycles the cells are adjusting their internal microstructures (like what happens in NiCd cells) to account for the loads that are put on them. Do you have any data (or would you be willing to do a test?) with maybe 5 cycles where you do not use the Triton to do the discharge, but rather put on the full load like you will use when you fly. I put a good ampmeter in series with the Triton to see how well they agreeded during discharge. They didn't! But I later came to realize it is the problem with the internal impedance of the DVM compared to the Triton. But what was strange was the "pulsing up and down" that my DVM showed during the Triton discharge. It made me really afraid to trust the 1.5 amp discharge rate. Maybe the A/D converter inside the Triton is really doing a cycle job to try and average out the current drop. Or maybe the cells jump up in down in discharge capability during discharge.

Anyway, it would be good to know if after the initial 3-4 cycles of "burn-in", does the cell imbalance remain. I have not seen it, but I have no cells bigger than 1200maH (except for the 2200 CBP group that ballooned).

By the way, are you aware of the construction of Li-poly cells? They are really 15-35 individual little cells wired in parallel by their metal tabs of copper and aluminum. The Sony Li-poly is one long roll of copper and aluminum with graphite matrix of Lithium and additives "smeared" to each side of the plates. The Sony cells are then folded over like a "mummy wrap" to get them to fit into the needed final cell package. Anyway, all the others I have disected are just a pile of individual cells wired in parrallel by their individual tabs. So any micro cell imbalance will be averaged out by all the other micro cells in the pack. I sort of think the same thing happens when we wire cells in series. There will be little differences in each microcell, but the aggregate will tend to balance out to the "weakest link". If that link goes "down hill" fast, then the pack fails. But most of the failures I have seen have the "weakest link" going to high impedance, not to a short condition. If the internal micro tabs happen to rub against another tab with a higher potential, it is possible to get small "ground loops" inside the pack. Charging the pack "very slowly" every once in a while tends to take the micro cell differences closer to zero. Thought I would throw that in, for what its worth.

Ted Cooper

Been following this closely Ted.

My flying pattern is usually 2-3 flights, then long breaks between...do you think frm what understanding you have, that fdropping teh charge rate frm ~1C to maybe ~0.25C when you just want to recondition the pack would be a good idea...or is it that if a cell is way out of line, you are going to overvoltage the others anyway?

Or to put it another way, what happens if you continue to charge past the normal terminal voltage, but at a much lower current?

Anyone?