John in CR
100 TW
The problem with the RC lipo is the chemistry, because it so easily goes into thermal runaway.
Reminder: Don't trust Hobbyking batteries.
- Thread starter Arkz
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Sunder
10 MW
Arkz said:
It's not an answer, it's merely a mitigating control. Under non-ideal conditions, LiPo can delaminate and gas. The gas is flammable, and the delamination is irreversible. So putting the cells under pressure can move the point at which these processes can occur. Someone smarter than me will have to explain why. Logically it seems fair enough to me though.
John in CR is correct in saying that RC LiPo's chemistry goes too easily into thermal runaway... But moving to 18650 or other cylindrical cell doesn't solve that issue. There's at least 5 different types of Lithium Ion chemistries that go into 18650, and one of them is LiCo - the same "dangerous" anode that goes into RC LiPo. That doesn't mean the electrolyte is the same though - the solid polymer burns better, but that certainly doesn't preclude a fire from starting in LiCo based 18650s. E.g. the ICR named cells.
18650 is just a format, it says nothing about the volatility of the contents.
I don't have pics of any of them anymore, but there's plenty on google:
https://www.google.com/search?q=thermal+runaway+in+agm+batteries&num=100&newwindow=1&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiN3tzxxJHcAhVtCjQIHYiMC4IQ_AUIDSgE&biw=781&bih=798
https://www.google.com/search?q=thermal+runaway+in+agm+batteries&num=100&newwindow=1&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiN3tzxxJHcAhVtCjQIHYiMC4IQ_AUIDSgE&biw=781&bih=798
flippy
1 MW
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Sunder said:
technically you are correct but you are baltingly ignoring a few facts while mentioning them yourself.
1: pouches to thermal because they delaminate easy because there is nothing to keep them from delaminating exept some heat shrink or something that doesnt do much.
2: 18650's have by their very design a compression housing. they cant delaminate. hence the problem does not apply no matter the chemistry used unless there is a MASSIVE fault going on, usually external. and at that point they vent in a specific direction. a LOT better and can be taking into battery design like tesla does compared to pouches that basically are a party balloon filled with fire and death.
3: follow up on number 2, can we please stop focussing on chemistry, it is getting annoying and it doesnt matter in the slightest. even the safest chemistry can go thermal. this discusssion is about how inherently unsafe uncompressed pouches are. not the chemisty inside them.
4: 18650's have smaller capacities. so if a cell goes it is a lot less eventful then a 20+Ah pouch that goes full on angry femminist on your work bench or worse: vehicle.
even seen a 30Ah pouch go thermal? quite something you dont want to be close to, especially not if there are a bunch of them in a pack without any insulating sheets between them. a 18650 venting (with or without fire) is a LOT less unnerving even inside a big pack because it wont take the rest of the pack with it.
Alan B
100 GW
Pouches require compression, it is a battery packaging necessity. One of the reasons it is hard to buy commercial pouches is they don't trust random customers to properly constrain them, so they only sell the high end units to verified clients. 18650's are one solution. People who think 18650's are magic are not well informed. They are a commodity product that lowers prices, but plenty have burned, and the packaging is not efficient for large batteries and brings additional risk due to the extensive interconnects needed. Even Tesla is moving away from 18650's. Most electric car manufacturers use larger cells or pouches.
All high performance rechargeable lithium batteries are highly flammable. Solvents and high energy density make a fiery cocktail. Even the highest quality manufacturers have occasional fires.
Take care, be vigilant.
All high performance rechargeable lithium batteries are highly flammable. Solvents and high energy density make a fiery cocktail. Even the highest quality manufacturers have occasional fires.
Take care, be vigilant.
Sunder
10 MW
flippy said:
Not ignoring them at all. In fact, I've already mentioned them.
Perhaps it's just a weighting of risk issue that causes us to disagree?
A properly packed pouch cell is no more dangerous than a properly packed 18650. But on the one hand, you have amateurs making cells out of RC LiPo and heatshrink... (Or gaffer's tape). On the other hand, you have scam artists making 18650s in a dirty lab in rural China...
Alan B
100 GW
The RC lipo packaging seems to meet the compression requirements just fine. Their market requires minimum cost and minimum weight. Their design meets the requirements of their market. It is up to the consumer who chooses these for their project to provide any additional requirements for their application.
flippy
1 MW
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but it doesnt. they dont last long, tend to blow up when actually loaded to their ratings and have systemic puffing issues. all off those things do not apply to properly compressed pouches like the ones you find in cars or just 18650 cells.Alan B said:
you would have to agree that hobbykings heat wrap is grossly inadequate for the job it has to do (apart from sporting flashy decals and marketing wank) of giving EQUAL pressure to the cells (wich it physically can't do) and is just there to keep the cells from flopping about.
there is a reason car makers have to resort to massive steel and aluminium plates, huge PA66GF30 support structures and steel bolts to keep shit from puffing and then you got a couple morons think they can strap the same pouches with some heat shrink to their bikes and dont expect stuff to catch fire as some bloke on the internets told them its safe because he cant beleve hobbyking would sell shit that is not dangerous....
nothing personal mate but your beliefs about what is safe is grossly influenced by marketing wank or the general misdirection given by manufacturers that claim their stuff is safe.
Sunder
10 MW
I think it depends on the brand/series and the usage.
I found a research paper a while back, that said that either 2psi or 2 pounds total of pressure was sufficient to prevent delamination, don't remember which exactly. From that paper, there were a lot of people who decided that heavy plates with threaded rods were overkill, and a G-Clamp to get the initial compression, followed by a non-stretching fabric based tape would be enough (A plastic tape would stretch over time).
If that's the case, heat shrink would also likely be "good enough" for some applications.
However, I think in the race to get prices down, but also a de-focus from RC car racing, to long flight times in drones, batteries have become lower and lower C rated to favour energy density over power density. The closer you run to the C rating, the more likely you'll need good compression.
I found a research paper a while back, that said that either 2psi or 2 pounds total of pressure was sufficient to prevent delamination, don't remember which exactly. From that paper, there were a lot of people who decided that heavy plates with threaded rods were overkill, and a G-Clamp to get the initial compression, followed by a non-stretching fabric based tape would be enough (A plastic tape would stretch over time).
If that's the case, heat shrink would also likely be "good enough" for some applications.
However, I think in the race to get prices down, but also a de-focus from RC car racing, to long flight times in drones, batteries have become lower and lower C rated to favour energy density over power density. The closer you run to the C rating, the more likely you'll need good compression.
Hillhater
100 TW
Actually, the results of millions of RC packs sold suggest that compression beyond standard shrinkwrap, is not essential.flippy said:
If it was, then all RC packs would swell and puff eventually, but they do not.
..and in my experience , the only ones that do puff are either the badly made ones (Quality failures) , damaged ones, or user abused ones ..over discharged, or overcharged , shorted, stored t full charge, etc etc.
The % of pouches that swell for no traceable reason is tiny
You cannot ignor the fact that there are millions of RC Lipo pouches in nothing more than shrink wrap, lying about in garages and hobby dens worldwide, and we hear very rarely of unprovoked/unexplained puffing failures.
Further i dont see how a 18650 cell can be said to be compressed , when in order to assemble it they have to slide the "jellyroll". Into the preformed can..IE ,..it needs clearance to fit. Do they even "shrink wrap" the jellyroll before its put in the can.
There has been some simple tests that have suggested that there is a little more capacity to be obtained from compressed pouches , but i have never seen any serious controlled tests to indicate extended cell life (most cells simply lose capacity progressively with cycling.
Lipo pouches should be considered dangerous generally, but that is mainly because they contain high amounts of energy, in a not too well protected package....but any cell type can burn and explode if mistreated.
jonescg
100 MW
You guys speak of pouch cells as if expansion is normal - it's not! If a pouch cell is puffing it's because something has gone wrong - either an internal short, physical trauma, over charge or over discharge, or the cells were allowed to get too hot for too long.
And yes, any lithium chemistry which has a metal oxide (LiCoO2, LiNiMnCoO2, LMO etc) has the potential to release molecular oxygen at high (read catastrophic) temperatures, and therefore tend to enter thermal runaway at about 140 to 160 degrees C.
Only LiFePO4 doesn't have this property, but it will still enter thermal runaway at a high enough temperature. The fire isn't as angry at least.
Cylinder or pouch - doesn't matter. The potential for fire is the same. Why so many Hobyking LiPo fires? Because Hobbyking. The QC is very ordinary, but the price is good. The two are inversely related.
And yes, any lithium chemistry which has a metal oxide (LiCoO2, LiNiMnCoO2, LMO etc) has the potential to release molecular oxygen at high (read catastrophic) temperatures, and therefore tend to enter thermal runaway at about 140 to 160 degrees C.
Only LiFePO4 doesn't have this property, but it will still enter thermal runaway at a high enough temperature. The fire isn't as angry at least.
Cylinder or pouch - doesn't matter. The potential for fire is the same. Why so many Hobyking LiPo fires? Because Hobbyking. The QC is very ordinary, but the price is good. The two are inversely related.
Sunder
10 MW
jonescg said:
Normal? No. Common? Yes.
My understanding is that when cells get hot - and all cells do to some degree (since all cells have internal resistance), the electrolyte - even solid polymer electrolyte is more likely heat and decompose, causing delamination and gassing. But just like water under a small amount of pressure has a higher boiling point, so too a small amount of pressure will increase the temperature at which decomposition will occur. (That second part was me guessing, I've never actually read anything about the specific mechanics of it).
So, in effect, compression is raising the C rate of the battery.
The thing is, manufacturers - especially cheap ones like sold on hobby king, list impressive numbers to sell more, and hey, if you kill them after 30 cycles instead of 300, more sales for them! I think compression raises both safety and life of the cells.
FWIW, EIG's NMC 20Ah pouch cells, in their own EIG-supplied cell-management hardware, do not have compression applied across the surfaces of the cells.
They have flimsy plastic frames that support the edges of the cells to keep the terminal tabs aligned, and which have nutserts under the tab area to allow bolting contacts, busbars, etc to them, with a thin aluminum separator between each cell. The center of the cell, very large surface area, on the plastic side, does not touch anything at all.
I only have experience with a few dozen cells (all in this hardware) so far, and only 18 of them for more than a couple of years. I don't run them near their max continous currents or near their peak rates, though on the first pack they would see relatively frequent momentary peaks near or at that max continous rate (at one point with one set of controllers, even just beyond that, but not approaching the peak rate).
The only cell that's ever expanded in any visually-detectable way was drained to reverse voltage by accident, in a 4s lighting pack, at low current (a few amps); the other three cells didn't get reversed but they did get drained farther than they should be (but still work even now, still as a low-current lighting pack). IIRC this was the same lighting pack accidentally drained to basically zero a few years before that, and recovered by slow-recharge.
They have flimsy plastic frames that support the edges of the cells to keep the terminal tabs aligned, and which have nutserts under the tab area to allow bolting contacts, busbars, etc to them, with a thin aluminum separator between each cell. The center of the cell, very large surface area, on the plastic side, does not touch anything at all.
I only have experience with a few dozen cells (all in this hardware) so far, and only 18 of them for more than a couple of years. I don't run them near their max continous currents or near their peak rates, though on the first pack they would see relatively frequent momentary peaks near or at that max continous rate (at one point with one set of controllers, even just beyond that, but not approaching the peak rate).
The only cell that's ever expanded in any visually-detectable way was drained to reverse voltage by accident, in a 4s lighting pack, at low current (a few amps); the other three cells didn't get reversed but they did get drained farther than they should be (but still work even now, still as a low-current lighting pack). IIRC this was the same lighting pack accidentally drained to basically zero a few years before that, and recovered by slow-recharge.
IIRC the 2psi spec is from A123 AMP20 pouches. I'm also fairly sure I've seen data of compression Vs. cell I.R.
I.e. compression reduces cell resistance, which improves performance and will reduce heating and so increase life.
Can cells will provide some compression as the can restricts expansion once the jellyroll starts to swell. By saying "well it's only a sliding fit when assembled" we're also assuming that there's no permanent growth during the subsequent formation. You've got to imagine the cell stack "breathing" as it's alternately charged & discharged as ions move in and out of the molecular structure.
Heatshrink, and anything without a stiff spreader plate only applies force to the edges, which is as likely to cause crush damage as anything helpful.
As to whether the heavy-duty arrangement seen on automotive OEMs is necessary, well these guys are demanding as much cycle and calendar life as possible from the minimum amount of battery (size/weight/cost) possible, so the cell manufacturer is going to stipulate optimum conditions for the cells, so hermetically sealed enclosure, low vibration, ideal compression, temperature control. As stated above, the HobbyKing heat shrink can be considered adequate for the application, because expectations are so low that any old crap will do.
I.e. compression reduces cell resistance, which improves performance and will reduce heating and so increase life.
Can cells will provide some compression as the can restricts expansion once the jellyroll starts to swell. By saying "well it's only a sliding fit when assembled" we're also assuming that there's no permanent growth during the subsequent formation. You've got to imagine the cell stack "breathing" as it's alternately charged & discharged as ions move in and out of the molecular structure.
Heatshrink, and anything without a stiff spreader plate only applies force to the edges, which is as likely to cause crush damage as anything helpful.
As to whether the heavy-duty arrangement seen on automotive OEMs is necessary, well these guys are demanding as much cycle and calendar life as possible from the minimum amount of battery (size/weight/cost) possible, so the cell manufacturer is going to stipulate optimum conditions for the cells, so hermetically sealed enclosure, low vibration, ideal compression, temperature control. As stated above, the HobbyKing heat shrink can be considered adequate for the application, because expectations are so low that any old crap will do.
Ianhill
1 MW
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The cells I posted as failed where in compression with alu plates.
All I done was remove the shrink wrap and then compress the complete brick, not the best way of doing it but when the cells are tab welded and soldered together from hobby king they do not come apart with out a major fight and sometimes damage to the tab so I kept them as blocks and compressed them slightly.
Helped keep the back temp down in summer but it increased sag in colder conditions, mind the failed cell had barely puffed at all and was in direct contact with the plate so I bet the damage was done with the cold saggy conditions the cell in question had direct contact with the alloy so it's temp should have been one of the lowest not good on a cold day even if I preheat the pack by time I discharged around 1 hour later.
It's for this reason you see active cooling with liquid rather than passive cooling with fins as the cell could be worse off in winter and warming a alloy fin that has air flow is not going to be anywhere near as efficient as a heat pump and a liquid system like a tesla for example.
Next I will try to compress with either wood or a custom vacuum formed case something that is a good insulator of heat, and a temp probe to trigger air flow through the pack at a set temp so I can keep warm and cool In a small compact area like a stand up scooter or eboard, ebike whatever.
All I done was remove the shrink wrap and then compress the complete brick, not the best way of doing it but when the cells are tab welded and soldered together from hobby king they do not come apart with out a major fight and sometimes damage to the tab so I kept them as blocks and compressed them slightly.
Helped keep the back temp down in summer but it increased sag in colder conditions, mind the failed cell had barely puffed at all and was in direct contact with the plate so I bet the damage was done with the cold saggy conditions the cell in question had direct contact with the alloy so it's temp should have been one of the lowest not good on a cold day even if I preheat the pack by time I discharged around 1 hour later.
It's for this reason you see active cooling with liquid rather than passive cooling with fins as the cell could be worse off in winter and warming a alloy fin that has air flow is not going to be anywhere near as efficient as a heat pump and a liquid system like a tesla for example.
Next I will try to compress with either wood or a custom vacuum formed case something that is a good insulator of heat, and a temp probe to trigger air flow through the pack at a set temp so I can keep warm and cool In a small compact area like a stand up scooter or eboard, ebike whatever.
