Although I’d manage to damage two LiPo packs over the last 18 months (one in a crash, the other by overdischarging) yesterday was my first experience with an in-flight failure. Since May of this year, I’ve had over 80 flights on my HET F-18 on 7s iRate 2200s (Mega 1615/5, Wemotec MF-480) without incident. I normally get two 4 ½ minute flights from a charge, don’t draw more than ~19A WOT, never saw an after-flight temperature reading hotter than 122F.
I’d flown the plane last Wednesday, and noticed that the power seemed down on the second flight; I landed early, figuring I’d pushed it harder on the first flight than I’d thought. Interestingly, pack only took 1439 maH on the charger. Since last week was the first cold snap of the season, I attributed the low power to the lower ambient (~45F), which was probably a mistake in retrospect—I should have checked the cell balance, even if it meant pulling off the shrinkwrap to get at the tabs.

Yesterday afternoon I launched my F-18, noted a slightly lower power output than normal (more a flop at the top of a reverse Cuban-8 than normal), but initially figured it was the ambient temperature, and would sort itself out as the pack self-heated. Flew for about three minutes, half WOT, half at ~3/4 throttle, when I noticed something trailing behind the plane, and it wasn’t my antenna! There was a thin trail of smoke that would have been perfectly acceptable from a four-stroke, but _way_ out of place for an EDF. I chopped power and landed ASAP, made the runway in good order, then things got much worse. Walking over to retrieve the plane, thick grey smoke started pouring out the battery-cooling intakes and exhausts. I picked the plane up by the wingtip and took it off the active runaway, yelling to ask if anyone had a fire extinguisher. One of the heli guys ran to get one from his car, and I pulled the canopy to try to get at the battery pack. Someone had the presence of mind to bring over a pair of pliers, and we were able to yank the smoldering remains of the pack out—whichever cell failed first started a chain reaction, and they were all toast, sending a thick cloud of smoke drifting down the flight line.

My best conjecture at this point is that one or more cells became unbalanced, and the colder than usual temperature also caused the internal resistances of the cells to differ more widely than over the summer, so the cell(s) with the higher IR overheated. Don’t think this could have been due to a tab overheating, as that should have resulted in a complete loss of power and control (I was running a uBEC in this plane) and I had positive control right down to landing.

This is purely anecdotal, but I’ve noticed that my 7s TP2100 pack is much happier in cold weather (<70F), but gets way too hot (~140F) in hot-weather flying, whereas my iRate 7s pack was perfectly happy all summer and chose to fail on the first couple cold fall days. Anyone else noticed any brand preference for different ambient temperatures?

Here are some aftermath pictures, taken in my garage after I got home:


Steve Guty

Steve sory for your lose very interestin to now the reason of the fail.

Steve

When was the lastime U slow charged at , <1C to equalize your 7S pack ?

Probably never ?

lipo's don't equalize like nicads.

I've not seen anything to convince me that slow charging would balance LiPo packs; what I did do was check the cell voltages before assembling the pack (all within .01V) with the special aluminum solder and paste.
What I have also done on other packs made up from 2s and 3s components, was to check and cycle each component separately.

This pack was hardwired in series; like I said, in hindsight I should've taken off the shrinkwrap and cardboard, and checked the individual cell leads at the first sign of anomalous behavior.

As far as I know, this is the first instance of a LiPo pack actually burning (as opposed to puffing up) in the air, and is especially disturbing in that the cells weren't pushed beyond their advertised specs.
Also be very curious to hear from anyone who's experienced the same temperature-specific behavior--hate to think of having to keep separate summer and winter packs, but it'd be cheaper than burning up a pack every six months

Steve

But slow charging can't hurt a LIPO cell from reaching a SOC ( stable open circuit )voltage level, or can it ?, as long as it reaches the " RESTING VOLTAGE " of 4.2V per cell.

Trickle charging beyond the 'Resting Voltage" of a LIPO is not necessary and probably is detrimental

Originally Posted by hall woo

Steve

When was the lastime U slow charged at , <1C to equalize your 7S pack ?

Probably never ?

Originally Posted by hall woo

Steve

But slow charging can't hurt a LIPO cell from reaching a SOC ( stable open circuit )voltage level, or can it ?, as long as it reaches the " RESTING VOLTAGE " of 4.2V per cell.

Trickle charging beyond the 'Resting Voltage" of a LIPO is not necessary and probably is detrimental

I think you still have some more explaining to do. If English is a second language for you, just take your time and consult the dictionary frequently. You may have good thoughts, but they are not translating well into written English.

If anyone is giving advice to people on how to use Li-poly cells, which are known to cause serious safety problems if not handled correctly, then it's important that the advice be clear, concise and accurate.

Originally Posted by hall woo

Steve

But slow charging can't hurt a LIPO cell from reaching a SOC ( stable open circuit )voltage level, or can it ?, as long as it reaches the " RESTING VOLTAGE " of 4.2V per cell.

Trickle charging beyond the 'Resting Voltage" of a LIPO is not necessary and probably is detrimental

Thx for the input, but charging past the "resting voltage" of a LiPo is the fundamental definition of overcharging, no? And we all know that _is_ detrimental.

Originally Posted by sguty

I normally get two 4 ½ minute flights from a charge, don’t draw more than ~19A WOT, never saw an after-flight temperature reading hotter than 122F. ........ but I’ve noticed that my 7s TP2100 pack...... gets way too hot (~140F) in hot-weather flying

OW ! , a 7s pack up in flames, that must be a little painfull economically. Not to bust on you personally, but everytime I see a post about somebody with Lipos letting their packs get that hot I start thinking you guys must be nuts. That's right out there on the ragged edge (of technology or sanity, take your pick) as far as I'm concerned. Maybe next time you could at least try to make the best of a bad situation and put a little pan of Jiffy Pop popcorn in your plane before flying it, in case it does catch fire again at least it won't be a total loss.

sguty,
thanks for the pictures. This is the first recorded Li-Poly vent and flame in air that could not be attributed to a wiring breakdown that could cause the plane to catch fire independently of the battery fire.

The fact that all 7 cells did the dominos effect tells me that most of the cells were quite hot. What was your WOT amperage measurement for your system?

In relation to Hall's comment about slow charge of LiPoly: there is some truth to it. I have seen cells that were slightly out of balance (like .2 volts misalignment). Charging at 1/10 C brought them back with no bad side effects. I don't know if this is a general "truth" or it was just a side effect of "being lucky". In most cells that are .3-.5 volts out of balance, they are too far out to be trickled back. They have to be individually rebalanced. And that is just the beginning. If they were initially balanced and got to .3 volts of differential, then there is an excellent chance that one or more of the cells is a "weak sister" and will go out of balance again even under conservative charge and discharge practices. So once you see imbalance (assuming they were matched in the beginning) be very suspect of the pack. Check the cell balance on every other charge. And keep the "eagle eye" out for the first signs of swelling.

Heat in packs: I think 122 deg F is OK if the ambient was 90 deg F and there was a long rest before the cells were recharged. The problem with measuring cell temperature is that the "middle" cell is not given the opportunity to complain about his really high temp (like 170 deg F) when his neighbors are just showing 125 deg F to the heat gun. That is why I am such a fanatic about there being ventilation space between the cells. It costs nothing and gives so many advantages. The only negative is your pack has grown by 1/8" in thickness. For the "skilled", I suggest removing the heat shrink on commercial packs and putting toothpick spacers between the cells (this causes no problem to the tab end). Then put clear tape at the top and bottom of the pack. Leave the middle open to aid in ventilation. The added bonus is that you will see swelling easier if it starts to happen.

I know this adds a tiny amount of labor cost to making a pack, but I think the manufacturers do not appreciate how important this is. Their only concern is how many cells they can pack in a shipping container: false economy.

WARNING: especially with greater that 3S packs, if they start to smoke and flame, get clear of the area. Don't even try to save the plane! The fireball can be as big as a person! Unless you have safety gloves, goggles, and a mask....you could become a statstic. It just simply is not worth it.
Crazy Ted -- hoping my stupidity will make safer flyers of us all.

I note the pack was way up at the front of the plane,with very little cooling. And that you flew it (2x 4.5 minute flights) at average 6.5C discharge?

I suspect the real damage was done before the last couple of flights - a back to back pair of flights with a hot pack.....everyone is so keen to show how a pack can be discharged in 6 minutes dead, whereas my only heat related failure was a BEC circuit that was in the air for over 6 minutes. At the end of tht it simply got too hot. Longer flight times without cooling are giving the components a lot of time to heat up, and cells in the middle will not like it. Once one cell starts to gas, its only a matter of time...

I also note that you have had over 80 flights - not bad - and the cells were bought in May, which I think is the slightly less 'hot' Irates?

So, early technology, pushed hardish, in confined space pops after 80 cycles?

I think that is a very excellent data point really.

Originally Posted by tjcooper

In relation to Hall's comment about slow charge of LiPoly: there is some truth to it. I have seen cells that were slightly out of balance (like .2 volts misalignment). Charging at 1/10 C brought them back with no bad side effects. I don't know if this is a general "truth" or it was just a side effect of "being lucky". In most cells that are .3-.5 volts out of balance, they are too far out to be trickled back. They have to be individually rebalanced. And that is just the beginning. If they were initially balanced and got to .3 volts of differential, then there is an excellent chance that one or more of the cells is a "weak sister" and will go out of balance again even under conservative charge and discharge practices. So once you see imbalance (assuming they were matched in the beginning) be very suspect of the pack. Check the cell balance on every other charge. And keep the "eagle eye" out for the first signs of swelling.

Ted, I've read an awful lot of industry data about Li cells over the past few years, and have not seen a single reference to slow charging Li packs to equalize cells. The only references I've seen to slow charging Li packs are that there's a school of thought that slow charging all the time will result in less likelihood of the pack going out of balance in the first place. If anyone has found an industry reference to slow charging unbalanced Li packs to bring them back into balance, I'd sure like to see it.

Bottom line is, if it works consistently, it would benefit a lot of people to know about it. If it doesn't work consistently, the myth should not be perpetuated.

Posted by tjcooper
In relation to Hall's comment about slow charge of LiPoly: there is some truth to it. I have seen cells that were slightly out of balance (like .2 volts misalignment). Charging at 1/10 C brought them back with no bad side effects. I don't know if this is a general "truth" or it was just a side effect of "being lucky".

----------------
I have been crucified here for suggesting this also.
My logic is that LiPos are constructed the same as any other non liquid rechargeable cells. I feel that slow charging ,perhaps every 20-30 cycles is benefical
I realize that LiPolys and NiCd are different chemistry however they both contain paste material.
When charging NiCads :
When fast charging the material nearest the terminals will charge first and the cell material in the deep interior of the cell will be charged last.

Until I prove myself wrong I will continue to slow charge LiPos from time to time. It works for me.


Sorry for you loss and this shows that all flying sites should have at least a couple of buckets of dry sand at the ready.

Charles

Originally Posted by vintage1

I note the pack was way up at the front of the plane,with very little cooling. And that you flew it (2x 4.5 minute flights) at average 6.5C discharge?

I suspect the real damage was done before the last couple of flights - a back to back pair of flights with a hot pack.....everyone is so keen to show how a pack can be discharged in 6 minutes dead, whereas my only heat related failure was a BEC circuit that was in the air for over 6 minutes. At the end of tht it simply got too hot. Longer flight times without cooling are giving the components a lot of time to heat up, and cells in the middle will not like it. Once one cell starts to gas, its only a matter of time...

I also note that you have had over 80 flights - not bad - and the cells were bought in May, which I think is the slightly less 'hot' Irates?

So, early technology, pushed hardish, in confined space pops after 80 cycles?

I think that is a very excellent data point really.

Actually, the cooling was fairly reasonable--you can just see one of the two intake ducts (reversed largish pushrod exits) in one of the phots, and there was a large (1/2") passageway back through to the fan spinner area--the smoke had no problem gushing out either end

I was very conscientious about checking pack temps, and had left enough bare areas in the shrink wrap to look at the center cells, at least edge on, so I'm pretty comfortable there was not a huge thermal gradient. I also let them cool to within 10F of ambient before trying the second flight.

Agee eighty cycles is an interesting data point--my friends who still fly glow would probably run through 10 gallons at nearly $20 a gallon for 80 flights, so $150 for the 7 cells is still a relative bargain....

These iRates were advertised as 10C continuous when I bought them, so I didn' t think I was pushing them all that hard. I've got an older pack of TPs that have done over 150 cycles at the ragged edge of their rating (10C), have shown worse heat during the summer months, and are still flying well now that the temperatures are down in the 50s--be curious for any details of the difference in chemical details, if any, between TPs and iRates

Originally Posted by tjcooper

sguty,
thanks for the pictures. This is the first recorded Li-Poly vent and flame in air that could not be attributed to a wiring breakdown that could cause the plane to catch fire independently of the battery fire.

The fact that all 7 cells did the dominos effect tells me that most of the cells were quite hot. What was your WOT amperage measurement for your system?

In relation to Hall's comment about slow charge of LiPoly: there is some truth to it. I have seen cells that were slightly out of balance (like .2 volts misalignment). Charging at 1/10 C brought them back with no bad side effects. I don't know if this is a general "truth" or it was just a side effect of "being lucky". In most cells that are .3-.5 volts out of balance, they are too far out to be trickled back. They have to be individually rebalanced. And that is just the beginning. If they were initially balanced and got to .3 volts of differential, then there is an excellent chance that one or more of the cells is a "weak sister" and will go out of balance again even under conservative charge and discharge practices. So once you see imbalance (assuming they were matched in the beginning) be very suspect of the pack. Check the cell balance on every other charge. And keep the "eagle eye" out for the first signs of swelling.

Heat in packs: I think 122 deg F is OK if the ambient was 90 deg F and there was a long rest before the cells were recharged. The problem with measuring cell temperature is that the "middle" cell is not given the opportunity to complain about his really high temp (like 170 deg F) when his neighbors are just showing 125 deg F to the heat gun. That is why I am such a fanatic about there being ventilation space between the cells. It costs nothing and gives so many advantages. The only negative is your pack has grown by 1/8" in thickness. For the "skilled", I suggest removing the heat shrink on commercial packs and putting toothpick spacers between the cells (this causes no problem to the tab end). Then put clear tape at the top and bottom of the pack. Leave the middle open to aid in ventilation. The added bonus is that you will see swelling easier if it starts to happen.

I know this adds a tiny amount of labor cost to making a pack, but I think the manufacturers do not appreciate how important this is. Their only concern is how many cells they can pack in a shipping container: false economy.

WARNING: especially with greater that 3S packs, if they start to smoke and flame, get clear of the area. Don't even try to save the plane! The fireball can be as big as a person! Unless you have safety gloves, goggles, and a mask....you could become a statstic. It just simply is not worth it.
Crazy Ted -- hoping my stupidity will make safer flyers of us all.

Thx Ted! The WOT amps were 19.7 right off the charger dropping down a bit and stabilizing at 18-18.5. I'd seen a max wattage figure of 482 on my Whattmeter. I'd generally given the pack at least 20 mnutes before a second flight or recharge, and didn't fly or recharge until the pack was pretty close (<10F) from ambient. As I mentioned above, since I'd built this from bare cells, I'd left gaps in the shrink wrap to look at the center cells with an IR probe.

I'm thinking that even one bad cell in a confined space could trigger another, and there goes the whole stack. Interesting that the smoke really blossomed after the plane landed, and the cooling airflow stopped. Did not see any flame (or actual fireball) other than on the balsa after the cockpit hatch was removed (just lots of thick grey smoke), but I didn't attempt to remove the hatch and look inside until the fire extinguisher was right at hand.

Defintely made for an excitng afternoon--glad none of the other flyers on the line lost sight of their planes in the smoke!