Li-Ion protection technology and possible dangers

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NewBie

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This is the split-off discussion about Li-Ion technology from this thread:
http://candlepowerforums.com/vb/showthread.php?t=105967
bernhard


Geologist said:
No war started here Newbie - just want to make sure that hard cold facts are present - here are some more-

Look at the details on the post where you state that George had me quote stuff. George never has had me quote stuff. I would imagine that Mr. Bulk or one of the other mods made those edits to the 1st post as new announcements/info.

Ah, so it was Bulk or whomever, that makes sense. Your name is on the post, so I assumed it was you making the comments over there.


Geologist said:
Exlposion Protection? The batteries are kicking off on the Chameleon because the CR123 batteries can not sustain enough current on the higher levels (think regulated light). As current hits the ~1.5 C, then they shut down. The better the battery can maintain voltage (or if using larger cells), then the longer the battery will run. Both the light and the batteries have thermal protection - we pay for it just it case it works.

Okay. Now, question, do you know what actually causes the cell to shut down? It is the cell's protection mechanism. This is specifically designed to protect the cell if all else fails. You don't need to explain to me what a regulated light is, I do understand much better than many folks, how a switcher works, and how in this design the uC is sampling the current thru a sense resistor, to alter the switchers operating point at the FB node.

As the cell voltage drops, when you have a regulated output, the input current has to go up, in so you have enough power (think watts) comming in.

So, as the cell voltage drops, the input current goes up. This causes more current to flow thru the Li-Ion cells PTC (which is one of the last ditch protection mechanisms- short the vents that are designed to make the cell spew stuff at a slower rate than a totally violent explosion- and those don't always work either) (assuming you don't have a sealed cavity, like a water tight flashlight with o-rings). The higher current causes the PTC (Positive Temperature Coefficient- resistance goes up with temp) to start heating up.
These PTC protection devices have a knee in them, so that when you exceed a certain threshold, their resistance rapidly goes up, which causes the current to go down (the cell's temperature also adds to the heating of the PTC). This is the mechanism that is shutting down the cells.

That PTC shutdown mechanism is the last ditch effort to protect the cell before it vents. It is certainly **not** designed for routine protection. Anyone that is relying on this as the protection mechanism is certainly playing with danger.

This PTC mechanism is installed right under the + button of the cell.

There is another type I've seen which is actually a polyfuse. When these blow, they actually disconnect. They must cool down before "resetting/healing" to allow any current to flow. Most battery vendors get around this, and make them work like PTC resistors (since they are cheaper), by squeezing the polyfuse under pressure in the cell. This causes them to act more like PTC resistors, but causes a rather considerable variation from cell to cell. The amount of pressure put on the polyfuse causes more current to be able to flow, before the polyfuse kicks off. This pressure is not something that is easily controlled, nor will necessarily stay constant over time.

The other protection mechanism you mentioned is in the switcher chip itself. This protection mechanism kicks off way up there at 125C, which is certainly a highly dangerous area for a Li-Ion cell to be at.

Also note, he mentioned right off the LTC3441/3443 datasheet, that hitting this 125C temperature isn't something you want to do, as the chip could blow.

Now, did you realize that 130C is considered the actual thermal runaway point for Li-Ion cells? As the switcher chip, which is the flashlight's "thermal protection" is not thermally connected directly to the cell, plus as Georges80 mentioned, the chip is thermally relieved by vias into a broad ground plane on the backside, it would be fool hardy to rely upon this for thermal protection of the cells, since it isn't monitoring the cell's temperature.

Most manufacturers will recommend the cell stays below 60C. This is a sheet for the Molicel:
"Temperature: Discharge ˆ20 to +60° C Charge 0 to +45° C Storage ˆ20 to +25° C (occasional short excursions to +60° C)"

http://www.allbatteries.com/catalogue/o17.pdf


From another datasheet:
Operating Specifications- Temperature Range
-Discharge -20C to +60C

http://www.molienergy.com/specs/ICR-18650.H.pdf


LG 18650:
3.8 Operating Temperature Charge : 0 to 45C
Discharge : -20 to 60C

http://www.batteryspace.com/productimages/li-ion/186502400A2%20PS.pdf


Please, keep in mind that LG and Molicel are the top Lithium Ion rechargable cell makers on the market, and most cells don't come close to their standards.


Once a cell hits thermal runaway, even removing the current draw will not stop the thermal runaway.

Now what happens when the chip blows? It is very likely it will short out. This puts a short on the Li-Ion cell. You **really** don't want to be shorting out Li-Ion cells...

Neither of these protection mechanisms would be something I'd even consider a true protection mechanism. Both are designed to function in disaster situtations, and each have their issues. This is nothing like the ARC or HDS design where things are actually monitored and controlled, so things don't reach this dangerous situation in the first place.

Protected cells are one step better. They put additional electronics in/on the cell, that protect against overvoltage, undervoltage, overcurrent, overtemperature.

With higher end Li-Ion cells, like the LG Li-Ion, they add a porous plastic "screen/mesh" in the cell. This is there in case the Lithium starts to kick off. What it does is to melt, hopefully cutting off the anode and cathode current path. There still have been quite a number of Li-Ion rechargable cells that have gone off with external protection circuitry, PTC, and the melting mesh. Due to this, Molicell has added an additional high temp mesh, on top of the lower temp melt mesh, that melts at yet a higher temperature, in an attempt to yet further protect against explosions.

These Li-Ion rechargeable cells really are not something one should be trying to push for all they are worth!
 
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Codeman

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Re: Chameleon flashlight problems

Good summary, Icebreak!

Geologist said:
Newbie -

Thats great information about how protected batteries work - it really should be stickied somewhere as we normally do not see that information condensed into one post. Thanks.
...
I agree! That should be stickied. Thanks for posting that, NewBie.

If a cell's PTC kicks in, doesn't that pretty much break the circuit, so the cell couldn't be used anymore? All of my cells continue to work fine. I don't think the Chameleon relies on PTC, or any cell protection for that matter. I would think that would be very unreliable, given the variety of cells.

I've had early cut-offs from full-protected, partially-protected (at least, that's what I think Powerizers are), and unprotected cells. In all cases, the cells were still cool (100°F or less). Since I've seen it with unprotected cells, it has to be the Chameleon's board doing the shutdown on those cells. On protected cells, I think Geologist's explanation is accurate, but I'm not clear whether it is the cell's protection or the Chameleon that's doing the shut down. My guess is that in some cases with particular cells, it's the cell, in others it may be the light. I just don't know enough to say. Either way, there's no safety issue under these conditions.

Of course, it's a different story when dealing with 123 cells that can handle the current draw of an extended CT5/5 run. The Powerizer cells from BatterySpace are the only ones I have that can handle the current draw consistently. Of those cells that I've run past 2 minutes, they all ran 7-10 minutes. The Chameleon was quite warm, as were the cells. I don't know exactly how warm, maybe over 120°F, maybe not. I could still hold the cells without having to move them around in my hand, so maybe they stayed below. Now that I know these long runs on CT5/5 aren't good, I won't be going there again.
 
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NewBie

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Re: Chameleon flashlight problems

Codeman said:
I agree! That should be stickied. Thanks for posting that, NewBie.

If a cell's PTC kicks in, doesn't that pretty much break the circuit, so the cell couldn't be used anymore? All of my cells continue to work fine. I don't think the Chameleon relies on PTC, or any cell protection for that matter. I would think that would be very unreliable, given the variety of cells.

You are welcome Codeman.

If the PTC cuts in, it only does that for a short period of time, and then they work again, just until it cools off. Depending on the current, it can only take seconds for the PTC kickoff to occur, the whole cell does not actually have to get hot. The PTC recovery time will depend on how hot the cell is.


Codeman said:
I've had early cut-offs from full-protected, partially-protected (at least, that's what I think Powerizers are), and unprotected cells. In all cases, the cells were still cool (100°F or less). Since I've seen it with unprotected cells, it has to be the Chameleon's board doing the shutdown on those cells.

See my comment above.


Codeman said:
On protected cells, I think Geologist's explanation is accurate, but I'm not clear whether it is the cell's protection or the Chameleon that's doing the shut down. My guess is that in some cases with particular cells, it's the cell, in others it may be the light. I just don't know enough to say. Either way, there's no safety issue under these conditions.

Of course, it's a different story when dealing with 123 cells that can handle the current draw of an extended CT5/5 run. The Powerizer cells from BatterySpace are the only ones I have that can handle the current draw consistently. Of those cells that I've run past 2 minutes, they all ran 7-10 minutes. The Chameleon was quite warm, as were the cells. I don't know exactly how warm, maybe over 120°F, maybe not. I could still hold the cells without having to move them around in my hand, so maybe they stayed below. Now that I know these long runs on CT5/5 aren't good, I won't be going there again.

When you use the term protected cells, are you referring to the unprotected cells that have the button on the end (these are the ones with the PTC)?

Information on really good PTC devices for batteries, is linked below. These Raychem PTC devices are among some of the very best on the market, and alot of time and money was put into their development. Due to cost, these very high quality ones are only rarely used:
http://www.circuitprotection.com/04Databook/E_strap_(275-300).pdf


Or what are considered protected, that have a circuit board on the bottom of the battery? This board can be made out, as you see a 0.031" to 0.062" bump on the bottom of the cell, as the shrink wrap goes around it. This circuit protects against over-discharge, over-charge, over-current, short circuit, reverse charger, and also usually will have over temperature protection. This is in addition to the other "mechanical" protection.

These will have circuits similar to Texas Instruments UCC3952-1/2/3/4 chips on the board, found here (very few actually use the TI device though):
http://focus.ti.com/docs/prod/folders/print/ucc3952-1.html

These are among the safest cells one can find on the market. JSBurly spent considerable personal money to develop and bring these to the market in the R123 form factor. Unscrupulous folks overseas started selling these to other vendors, cheating Jon out of his rightful due.


There is also what is known as a bare cell, which has no positive button on the end. These are highly dangerous without more complex external electronics and temperature monitoring devices. These are not supposed to be sold to consumers, without additional protection devices/circuits added, and are designed for use in battery packs. There have been reports that some low ballers will take these and just stick a + nubbin on the end, under the shrink wrap, without the required PTC and other mechanisms. These are technically illegal for sale in the US, but that doesn't mean you cannot get them. Be careful where you get your cells from. The prices on these are a bit lower than the other unprotected cells. Read these as ***DANGER!!!***


The permanent thing that can "kill" the cell is called the CID (circuit interrupt device). This is designed to actually warp when battery pressures get too high during charging, and permanently disconnecting connection to the cell. It is a little dome inside that pops to the opposite dome shape. These do not reset. If you are kicking these off during discharge, whomever designed the cell really screwed up royally, and the PTC should have kicked off first, or it is possible that the PTC actually failed (which is not unheard of).

Edited to add further information.
 
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Codeman

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Re: Chameleon flashlight problems

NewBie said:
You are welcome Codeman.
When you use the term protected cells, are you referring to the unprotected cells that have the button on the end (these are the ones with the PTC)?
...
More good info! I didn't realize that there are two different things, though what I had in my mind as far as the term "protected" are the CID's. All my RCR123's have positive anode buttons. Those from AW also have CID's. I think those from BatteryStation have them (don't have any with me to check). The unprotected cells from LightHound don't, nor do the Powerizer cells from BatterySpace. I've always thought of the Powerizers as partially protected, since they used to claim overvoltage protection on the website. At least, they did at the time I bought them a year ago. That claim is now gone. It doesn't matter, though, since that only comes into play during charging.

FYI - Even though the Powerizer cells are the lowest rated (640mAh) of the cells I have, they seem to handle the high currents of CT5/5 better than the other 123's. Another intesting point - the website now refers to these cells as intended for industrial use only: http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=1389 . I _think_ that's due to the higher voltage as compared to primary 3V 123's. Is there a non-destructive way to verify the presence of a PTC?
 
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SilverFox

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Re: Chameleon flashlight problems

Hello Ray,

Your testing is what tripped the "safety" concerns for me...

First of all, let me say that Charlie (Mr Bulk) has been very innovative in offering bright, multi level, compact lights. As a person that is quite often in variable ambient light situations, including darkness, I appreciate the ability to "dial in" the amount of light needed at the time. The Chameleon offers a giant step forward for multi level lights.

When you reported your testing results, you indicated that the light was getting hot enough that you decided to cool it off by placing it on a gel ice pack. It has been my experience that most people can handle hot things in the 120F - 140F range with mild discomfort, but will still report it as being hot. You reported that you ran several cells through the light and it kept getting hotter. Please note that we do not have exact numbers here, so it is all subject to a bit of conjecture.

The information Newbie presented is correct. Let me expand it. Li-Ion cells have a highly volatile chemistry that starts to break down at higher temperatures. Safety mandated a way to protect against thermal runaway. The manufacturers came up with PTC protection. This is kind of a last line against excessive heat. When you bump up against it, it means that the internal cell temperature is too hot. When the internal cell temperature cools off, it resets, and the cell can be used again.

The problem is that the PTC device is tested for 1 cycle. There is very little information on how many cycles it can withstand. When you continue to run cells through your light and the PTC is tripped a number of times, you risk damaging this very important safety device. The testing done to comply with shipping restrictions dictate that a cell be shorted for a number of hours and show no adverse effects. They do not do multiple shorts to test the PTC, it is only tested for one use.

Please note that we are talking about internal cell temperature here. The outside of the cell may appear to be a lot cooler, while the internal temperature is higher. I have read reports of people checking "flash amps" of an unprotected Li-Ion cell and have noticed that it shut down during the testing. While the cell remains cool to the touch, the PTC has tripped indicating a hot internal temperature.

When the chemistry inside the Li-Ion cell goes above around 140F (60C), the cell suffers damage. This damage results in reduced cycle life and reduced capacity.

When the chemistry inside the Li-Ion cell goes above 194F (90C), you are at the ragged edge of thermal run away. At 266F (130C), thermal run away is immanent.

Once again, please note that we are talking about internal temperatures. Also, keep in mind that the heat/reaction curve is exponential. It may take a long time to get to the damaging temperature, but from there things progress rapidly.

A protected Li-Ion cell that shuts off due to low voltage cut off is less of a concern. The high and low voltage protection circuits are designed for cyclic use, however I am not sure how many cycles they are rated for. I would also expect that there may be a difference in the quality of these circuits that may be reflected in the cell price.

So where does that leave us?

Charlie is removing this level from the remaining lights, so it is a bit of a non issue now.

You have a number of bare Li-Ion cells that may have had their PTC protection exercised, but we don't know for sure. If we were in an industrial situation, I would suggest you dispose of those cells and start with new ones. However, we are in a hobby environment and "know" that the PTC is good for at least a few cycles, so I will leave it to your personal discretion.

It may be that this "turbo" mode would be better powered by the larger 18650 Li-Ion cells, but some testing would have to be done to confirm this.

Finally, when you do testing like this, I would recommend that you let things cool off between tests... :)

Tom
 

dat2zip

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Re: Chameleon flashlight problems

Codeman said:
FYI - Even though the Powerizer cells are the lowest rated (640mAh) of the cells I have, they seem to handle the high currents of CT5/5 better than the other 123's. Another intesting point - the website now refers to these cells as intended for industrial use only: http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=1389 . I _think_ that's due to the higher voltage as compared to primary 3V 123's. Is there a non-destructive way to verify the presence of a PTC?

The fact that Powerizer cells work is a warning sign. If this was a protected cell the PTC adds resistance internally to the battery. This sounds like an unprotected cell which does not have the PTC and thus won't sag as much delivering more power. It's the unprotected cells that are the most likely to explode.

I don't know this for a fact about the Powerizer cells. Just a word of caution here if the batteries you have are not protected.

Raychems polyfuse technology is very stable and very well understood. Unless there is another technology out there I'm not aware of the resistance of the polyfuse for a given trip current and voltage rating is pretty much a constant. I doubt that from battery manufacturer to battery manufacturer the specs of the battery internal resistance with a PTC would change much. The battery chemistry for li-ion is very low and the dominate resistance is the PTC.

Wayne
 

Codeman

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Re: Chameleon flashlight problems

SilverFox said:
...
You have a number of bare Li-Ion cells that may have had their PTC protection exercised, but we don't know for sure. If we were in an industrial situation, I would suggest you dispose of those cells and start with new ones. However, we are in a hobby environment and "know" that the PTC is good for at least a few cycles, so I will leave it to your personal discretion.

It may be that this "turbo" mode would be better powered by the larger 18650 Li-Ion cells, but some testing would have to be done to confirm this.

Finally, when you do testing like this, I would recommend that you let things cool off between tests... :)

Tom

Hi Tom,

I appreciate the additional info and warnings. From the field testing of bwaite's USL (long runs with head temps reaching 165°F+ using a DMM), 145-150°F is where I start moving something around in my hand to keep it from hurting. That's about where the light was on the last 3-4 cells, which were the Powerizers. I put the light on an ice pack for 2 or 3 of the last cells, it was during the first one that the light got hot. It had been warming up slowly over all the previous cells, but the temp difference was great enough that I'm confident those were under 120°F. I'm confident enough with these temp guesstimates for myself, but I wouldn't expect anyone else to.

If I'm understanding things, it would seem that the highest level of safety would require throwing out any 123 rechargeable cell, regardless of whether it's protected or not, if the cells reachs 140°F internally or so, right? I know that, externally the cell was warm, but not as hot as the light. Since we can't measure the internal temp, just how do we decide when it's time? I mean, do you have any guidelines on how you decide, that you'd be comfortable with sharing?

dat2zip said:
The fact that Powerizer cells work is a warning sign. If this was a protected cell the PTC adds resistance internally to the battery. This sounds like an unprotected cell which does not have the PTC and thus won't sag as much delivering more power. It's the unprotected cells that are the most likely to explode.

I don't know this for a fact about the Powerizer cells. Just a word of caution here if the batteries you have are not protected.
...

The operative word being if - and I now have doubts that they are.

To be safe, then, any cell whose PTC might have been tripped, should be viewed as suspect - especially so if the cell got hot, right?
 
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Bullzeyebill

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I would bet that the Powerizer cells are not protected in any way, but I also believe that cells can not really support the highest level in the Chameleon, 1200mA's for any amount of time, and this would probably vary, led to led, circuit to circuit, and battery to battery. The Chameleon circuit would recognize this as minimal voltage to run the 1200mA setting. CT5 level 5, and would shut down. Codeman's testing may have stressed his cells, we can only guess, and it is probably a good idea to replace them. I still believe that the Powerizer cells are the best for the Chameleon, and will be good for CT4 level 5, 1000mA to led.

Bill
 
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SilverFox

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Re: Chameleon flashlight problems

Hello Ray,

Codeman said:
Hi Tom,

I appreciate the additional info and warnings. From the field testing of bwaite's USL (long runs with head temps reaching 165°F+ using a DMM), 145-150°F is where I start moving something around in my hand to keep it from hurting. That's about where the light was on the last 3-4 cells, which were the Powerizers. I put the light on an ice pack for 2 or 3 of the last cells, it was during the first one that the light got hot. It had been warming up slowly over all the previous cells, but the temp difference was great enough that I'm confident those were under 120°F. I'm confident enough with these temp guesstimates for myself, but I wouldn't expect anyone else to.

If I'm understanding things, it would seem that the highest level of safety would require throwing out any 123 rechargeable cell, regardless of whether it's protected or not, if the cells reachs 140°F internally or so, right? I know that, externally the cell was warm, but not as hot as the light. Since we can't measure the internal temp, just how do we decide when it's time? I mean, do you have any guidelines on how you decide, that you'd be comfortable with sharing?

140F is where damage starts, 194F is where you could end up totally destroying the cell, or at least doing major damage to it. The battery manufacturers list performance out to 80% of full capacity. My general procedure is to establish a base line performance, then use the cell until it drops below 80% of the base line.

Codeman said:
To be safe, then, any cell whose PTC might have been tripped, should be viewed as suspect - especially so if the cell got hot, right?

In the strictest sense, yes. On the other hand, I have cells that have gotten hot, and continue to use them with caution.

Tom
 

Bullzeyebill

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I have five Powerizer 3.6volt, 650mAh cells I bought last year from Battery Space. They all have a postive button, same look button as my Battery Station cells.

Bill
 

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Here's some more from another thread:

NewBie said:
You are flirting with some serious crap here, a simple fusable link most definitely is not sufficient for bare cells!!!


Please folks, if you don't know exactly what you are doing, these are not something one should mess with.

You most definitely **DO NOT EVER** put these in series, without electronics. If one of them has a lower capacity, then the two with the higher capacity will cause the weaker one to be over discharged, which is very dangerous.

With stuff like this that is highly dangerous to life and limb, it would be a really **good** idea if people that do not know exactly what they are talking about, not to pretend to be smart.

Please fellas, I'm very serious here. It would really suck if someone lost an eye, a few fingers, or who knows what else, because of some ***really dumb*** advice.

There are a few folks here I'd jump right in their face, DI style, and rip them a new one, if they were standing in front of me, over some of the advice offered so far.

If you are going to try and get away with just a fusable link on bare cells, practice tossing your maglite in well under 0.1 seconds, purchase a kelvar vest, full face shield, know how to dig a foxhole for protection, and learn how to yell **GRENADE!!!**, and/or learn how to dig a grenade sump in a foxhole.

I have actually seen D sized LiIon cells, rip apart aluminum shells (thicker than a maglite), like wrapping a firecracker in aluminum foil.

For Heaven's sake, Please be careful!!!


NewBie said:
[size=+2]For Heaven's sake, Please be careful!!![/size]

Commercial Li-ion battery packs contain multiple redundant protection devices to assure safety under all circumstances.
-a MOSFET opens if the charge voltage of any cell reaches 4.30V
-a MOSFET opens if any one of cells discharge voltage goes below 2.5V
-a fuse activates if any cell temperature hits 65-90°C
-a MOSFET opens if the current threshold gets higher than 2C
-a MOSFET opens if any one cell's temperature reach 65-90°C
-a mechanical pressure switch in each cell permanently interrupts the charge current if a safe pressure threshold is exceeded
-a PTC opens if cell current reaches ~2C

Most of these mechanisms are not present in bare cells, and are illegal for sale as bare cells in many places around the world, to consumers.


Venting- Release of internal pressure from a cell by ejecting some or allof its internal components into the environment
–These components may be flammable and may include noxious gasses
•Li/SO2battery releases acutely toxic and flammable gases
•Li/MnO2battery releases flammable gases
–A venting of a lithium ion battery may release
•Flammable organic electrolyte (e.g. PC-EC-DMC)
•LiPF6--this material is reactive with water; forms Hydrofloric acid (nasty stuff)
•Carbon either as carbon or water reactive lithiated graphites
•LiNiCoO2 or other lithiated oxides and heavy/transition metals
•Metal foils and fragments (copper or aluminum)
•Methane, hydrogen, carbon monoxide (electrolyte decomposition

Ventings may be accompanied by smoke, sparks and or flames!
-Vents do not always work, and sometimes plug, generating tremendous pressures
-Cell defects or abuse may result in ventings that may be high pressure events producing shrapnel


Please do visit this link and pay attention to the photographs of these puppies going off (and remember these are smaller cells than these D sized ones)
http://proceedings.ndia.org/5670/Li...-Winchester.pdf


Here Valence has a really cool video, where they get to a safe distance and shoot a bullet at a Li-Ion pack, to simulate damage or abuse, or a protection mechanism failure:
http://www.valence.com/SafetyVideo.asp


These puppies are dangerous enough when protected via multiple means. To learn about some of the dangers even protected cells pose, please read over this thread (and remember the discussion is about protected/unprotected cells- not the bare ones sold here->which are even more dangerous):
http://candlepowerforums.com/vb/showthread.php?t=106242
 

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greenLED said:
:eek: the explosion/venting/fire on that vid is a real eye opener! :duck:

There are lots of those Li-Ion videos out there, and plenty of examples of people hurt by them. Here is a little more info, I'll quote myself, so the information is in this thread:

NewBie said:
Notice how they will put a polyswitch device in each cell, plus another in-line, when building up a battery pack:
http://www.molalla.net/~leeper/lithiu~2.pdf

The ones that they put on bare cells, then this form a unprotected cell. This allows the device to protect the individual cell, and are designed to trip at a lower and lower current as the cell heats up. Additionally they put another outside the cell.


Then there is the next layer of protection, which is put on the outside:

In a battery pack situation, then additional electronic circuitry is added on the outside, like what is shown here:
http://www.bourns.com/pdfs/AppNotes_LILP.pdf

An example of a external battery pack protection circuit chip is found here:
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3950/ln/



To form an individual Protected Cell (From a Unprotected-which also has the PTC, and the other various forms of protection):

An example of a chip used for an individual cell, to form a final protected cell is found here:
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3471/ln/
This chip gives you:
-Overvoltage Protection
-Overcurrent
-Short-Circuit Protection
-Undervoltage Protection
-Overtemperature Protection
 

NewBie

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Now lets move on to pushing a Li-Ion cell beyond it's design:

Operating Current Limits
Manufacturer's do not just randomly pick out ratings for their cells. Many Li-Ion cells datasheets will specify a *maximum* 2C discharge rate. If a cell is rated for 650mAH, then it's 2C rating would be 1300mAH. This is the *absolute maximum* rate you are supposed to discharge the cell at. This does not hold true for all Li-Ion cells. In some cases it is less. In all cases, it is inherently sane (and imho very important) to go and actually look at the manufacturer's datasheet for the cells maximum discharge rate.

So, just what happens if you push a cell too hard?

In a Li-Ion cell, if you push the cell beyond the maximum discharge rate, you in essence are pulling excessive current from the cell. This often results in permanent damage to the Li-Ion cell. It may be gradual, or it may be more immediate.

While discharging at high charging currents, the Li+ ion starts to diffuse improperly into the anode’s graphite particles which starts a plating of lithium metal on the surface. This plating caused by high discharge current forms in a manner which causes dendritic growth that can puncture the separator and form a conductive short circuit to the cathode. In some cases these will be "soft shorts", where the cell has a momentary drop out of voltage. In other cases it can result in a chain reaction which results in very violent venting, or possible explosion. Even a venting stituation can be dangerous, and Li-Ion cells can vent with flame and hot molten metals, and very reactive hot Lithium.

If you are pushing your cells beyond their ratings, sometimes you will just see a rapid reduction in cell capacity. If the cell doesn not explode or vent, your cell suffers an early death. This plated lithium metal mentioned above is quite reactive toward the cell's electrolyte and results in electrolyte decomposition at the anode, growing the thickness of the SEI layer. This will also increase internal resistance, and reduction of available electrolyte.

Another situation that happens when running Li-Ion cells beyond their ratings is called "polarization". Polarization, is the inability to move lithium ions through the electrolyte, and in and out of the active materials, will greatly reduce the cell’s performance.

When building Li-Ion cell packs, where the cells are in series, another technique is used during the build process, above and beyond all the aforementioned protection devices and circuit. In this case, battery pack manufacturers will actually run the cells over a number of charge and discharge cycles. Then they will match cells with similar characteristics. This helps prevent the over discharge (dangerous) of one of the cells in the series pack. A weak cell in a pack may also get warmer than other cells in the pack. Above and beyond all this, the associated electronics in a series pack actually monitor each cells individual voltage, and have various mechanisms to protect from explosion and venting of the weak cell.

The weak cell scenario also applies to the charging of the series pack. In this case, the weak cell may become fully charged before the rest of the cells. Then the cell starts to overcharge, which can result in venting and/or explosion. The electronics also protect against this scenario.

Unlike NiCd, Lead-Acid, or NiMH, there is no natural mechanism for Li-Ion cell
equalization during charging. This is another reason why the electronic protection circuits, and proper chargers are necessary. Otherwise you risk venting and/or explosion.

In situations where packs may be exposed to an unbalanced external thermal gradient, the cells will not discharge at the same rate. In this case, it is even more important to have the electronic protection circuitry. Otherwise, once again, you risk the vent/explosion senario. Though, the proper way to deal with this is not to put the cells in a situation where there is a thermal gradient.


I sure hope folks that are starting to buy some of the larger, and even more dangerous Li-Ion cells read and heed all the information presented so far.

Edited for spelling.
 
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cy

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good to see someone else broadcasting out these warnings...
 

SilverFox

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Hello Newbie,

Thanks for digging this information up.

Our safety record on CPF is very good, however as we begin to push the limits, it is very important to stand back a moment and review the safety concerns.

I have taken the time to review this information, as well as other information, and assess what I am doing from a safety perspective. I would encourage others to do this as well.

Tom
 

NewBie

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You are welcome fellas/ladies.

My hope is that nobody gets injured. The measure of success is that nobody is injured.

So, what do these mechanisms basically look like, and where are they in the cells?

Here is an unprotected cell (no electronics-electronics would make it a protected cell). A bare cell is lacking both of these mechanisms as is very dangerous to use as is.

liion2.png


liion1.png



A one of the warning that people do not often heed, but should also be mentioned.

Do not ever use the battery when it's temperature is/has reached 80 degrees C or higher. If the plastic resin separator within the cell is likely to become damaged due to overheating. Internal short-circuiting can occur, possibly leading to leakage, overheating, smoke emission, bursting, bursting with flames, explosion, and/or ignition of the battery.

Do not pierce the battery with any object, strike the battery, impact the battery, dent, or otherwise deform it. Cells subjected to any of these scenarios should be disposed of immediately, in a safe fashion.

Never use a Leaking Li-Ion battery. Dispose immediately in a safe fashion.
 
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