Li-poly batteries - incendiary device look out!!

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bobc
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Li-poly batteries - incendiary device look out!!

Postby bobc » Sat Jan 22, 2011 10:12 pm

OK the actual post is nothing like so inflammatory as the title...
I got my ebike project mobile today
Image
The cheap 40A ESC turned out NOT to have undervoltage protection & I discharged a 3S lipoly to 6.5V.
Oops
The side was quite swollen so I cut off the terminals and was about to bin it when the thought struck me - let's see what a lithium fire is like.
Took it somewhere safe & attacked it (gently) with an axe. (I wanted to puncture it, but didn't want to be very near it when I did).
Was there a resulting fireball? Not before I came home several hours later.
Image
next week I'll overcharge it with a car battery charger & see if I can ignite it that way...
I had read that a punctured li-poly would "fireball" within a few minutes.
This one was (over) flattened but there was no warming, smoke, smell or any other sign of doom.
Stand by for more updates later ;^)
Bob

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ChrisB
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Re: Li-poly batteries - incendiary device look out!!

Postby ChrisB » Sun Jan 23, 2011 10:10 am

Interesting, always like a bit of rough science 8)

I suspect the reason behind the fireballs etc is due to them being in a charged state, thus this one being some what discharged isnt going to have any energy in it to give up as a fire ball :?

Thats how I see it, unless its sheerly the chemistry of the battery that produces these fireballs :?

Wonder what the actual chemical reactions are :?



ChrisB
I reject reality and substitute my own !!!!!!

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Jeremy
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Re: Li-poly batteries - incendiary device look out!!

Postby Jeremy » Sun Jan 23, 2011 12:01 pm

The whole LiPo fireball myth has been elevated to almost cult status, now, it seems.

I spent some time trying to track down and separate genuine lithium battery fire cases from mythical ones to see just how probable it was that these cells would catch fire in normal use. The first thing I discovered was that the number of incidents is very small, just a few dozen worldwide. As far as I can tell, the total number of lithium battery sales runs into the hundreds of millions.

All of the incidents of cell problems on discharge that I could find related to very early lithium cobalt oxide cells with a low maximum discharge rate, being discharged at close to, or in excess of, the manufacturers stated maximum. None resulted in actual fires or explosions, but there seem to have been three incidents where cells got hot enough to vent and release fumes and vapour. Since the development of high discharge rate cells the discharge related incidents seem to have become a thing of the past. The worst that happens is that the cells die and may puff up a bit.

There are reported instances of fires caused by batteries being short circuited by physical damage, but these relate to all battery chemistries. Most battery explosions seem to occur with lead acid cells, most battery fires seem to occur with nickel and lithium chemistries. I think it's fair to say that any battery presents a big risk of fire or explosion if shorted out.

The vast majority of lithium cell incidents occur whilst charging. It's hard to be definitive, but it seems likely that defective chargers or charging at too high a rate has been the major cause of incidents. Since the near-universal adoption of chargers with a proper CC/CV charge characteristic these incidents have reduced dramatically.

The user group that place the greatest demands on LiPo type cells are the RC model users. They frequently discharge at rates of around 5 to 6C average and expect to charge at rates of at least 2 to 3C in the field. They rarely use cell-level discharge monitoring, instead relying on the very crude cut-off voltage that can be programmed in to most RC typoe controllers via the beep codes or programming cards. Many users leave their controller at the default setting (which is often a very low LVC) and don't bother with programming them, because it can be a pain to do on the cheap ones. The result is that incidents like Bob's are not that uncommon, yet they rarely cause any risk of damage, other than killing the battery pack.

A scan around the RC forums reveals that battery fires are often discussed but rarely witnessed or factually reported, other than people deliberately trying to set packs on fire, a bit like Bob's experiment.

All the evidence points to LiPo packs that use lithium cobalt oxide being reasonably safe and very unlikely to catch fire or explode if properly managed. Careful charge control is vital, and proper cell-level discharge monitoring prevents early cell failure. Cell level monitors are very cheap and effective, something like the $2 cheap monitor from HK (http://www.hobbycity.com/hobbyking/stor ... oduct=4175 ) or the more capable $12 one (http://www.hobbycity.com/hobbyking/stor ... oduct=8927) would have probably saved Bob's pack from cell failure.

Jeremy

bobc
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Re: Li-poly batteries - incendiary device look out!!

Postby bobc » Sun Jan 23, 2011 12:59 pm

I was keen to give a battery some serious abuse because I'm going to have a LOT of them in the landspeed record car & I need to know the risks/hazards and develop suitable strategies. I do feel somewhat less worried about the battery/fireball issue now but will carry on with the tests ;^)
We had a really big lead acid wet cell go pop in my lab at college, shorted by an MSc student (most dangerous animal on the planet...). It's the hydrogen/oxygen vented when overcharged that likes to pop on them - I toddled off to the chemy dept to get some bicarb & 2 weeks later noticed about 5 round holes appearing in my jeans - seems like I managed to collect some acid even though I was at the other side of the lab!
What do other folk do - the under voltage detection on my ESC clearly wasn't doing its thing (I'd programmed it for automatic detection on the beep codes - probably where I went wrong, should have said 6s...) - do people use a voltmeter display (that can't be any good - who's going to look at that when they're driving?) or should I put some sort of comparator circuit in the home- made bit of the electronics (seem most practical to me - in fact dead easy to do too - it has a 5V reference already in it...).
Oh yeah - the dodgy bit of home made electronics hanging from its wires & parcel tape in the top picture is a wee circuit to generate the PWM RC servo control signal and 5volts from a twist grip throttle (£5 ebay purchase). It's actually 2 ancient RS components 555 demo boards bolted together... If this would be useful to anyone else I can post the schematic - it would be very easy to make on veroboard. It seems to have all the dynamic range in the last 1/4 of twist, but that seems to be a feature of the twist grip rather than the electronics. The twist grip is an active hall effect type thing (to my surprise), not a pot, so should last reasonably well.

Bob

PS - I also did a trawl of lipo fires - most (listed) were from overcharging but 2 or 3 were "my dog bit it" which made me laugh..
PPS the bike worked REALLY well - just with bargain basement RC gear I had lying around. My Nephew has it now to use for an unforgiving commute over pendle hill (!) - he'll tidy it all up & put a more siutable ESC & bigger batteries on. It's a "skip rescue" bike (as you can see). The motor is FAR too big but worked perfectly with a 40A 6S ESC, so the drive system power was limited to (within shouting distance of) nearish legal levels... Max speed is set by kV of motors, battery voltage and gearing - in this case 12.5mph, which it would still do up a 1 in 7 hill.
Nothing got even warm!

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Jeremy
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Re: Li-poly batteries - incendiary device look out!!

Postby Jeremy » Sun Jan 23, 2011 1:46 pm

Bob,

The easy way to manage discharge is to use off-the-shelf monitors. I've used the $12 BM-6 for a 6S pack, it works very well and gives a loud warning if any cell drops below the discharge voltage threshold. For my motorcycle LiPo pack (which is 15S) I'm using three small LVC boards put together by a chap on ES, see here for details: http://endless-sphere.com/forums/viewto ... 31&t=21695 These boards are tiny (a 6S board is 26mm x 43mm)and can be stacked to build any size of LVC system you want, with the warning line simply connected from module to module. I will have mine connected to cut off the controller in the event of an under-voltage event, using the enable line, but the same technique can be used with any ebike controller by using the ebrake line they all seem to have.

The throttle problem with RC type ESC can be solved with a $6 servo tester, like this one: http://www.hobbycity.com/hobbyking/stor ... oduct=8296 or something similar from your local hobby shop (where it'll probably be around £5..........). They can be adapted to work with the cheap Chinese Hall throttles that are endemic on ebikes, or use the far more expensive (but very nice) pot throttles, like the Magura ones that are popular with motorcycle conversions.

The linearity of the Hall throttles is pretty good, I've tested two different types and they were both pretty linear over their operating range (which is about 1.2 to 4V when fed with 5V). My guess is that your 555 circuit may be a bit non-linear and the cause of the response problem. Most people using these Hall throttles with RC type controllers complain that the response is the other way around, far too sensitive at the bottom end. This is a function of the controller being a speed controller, not a power or torque controller, and seems an inherent issue with a lot of cheap controllers.

Jeremy

bobc
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Re: Li-poly batteries - incendiary device look out!!

Postby bobc » Sun Jan 23, 2011 4:16 pm

yeah an audio warning could be practical - still needs the driver to react appropriately though - & its an easy job to have the controller cut the demand on the servo output. Also something that looks at every cell sounds good (anything just looking at total volts won't do that)

The twist grip that Matt got (ebay fiver) has "Longma" written on & its output goes from 100 to 400mV with (as I said) most of the movement happening in the last 1/4 of the twist. It's fine on the road & easy enough to modulate. Having used "pot" style controls in the school racing cars for many years, I would frankly use anything else in preference - the pots were always unreliable, particularly after winter storage. That actually cost us a few races until we replaced them with switches. So my prejudice would be to use a cheap hall effect over an expensive pot any day ;^) I'm sure there are "good" pots but it's always a mechanical double slidey contact & my experience says avoid!

Yes we used a "servo tester" thing from Hobby city for initial drivetrain testing & it would have been feasible to swap the pot on it for a pot based twist grip, but it was much easier to just make a new circuit and take advantage of what looked like a far better twist grip technology. As an added bonus it also gives us the 5V we needed for the ESC interface (it's non- BEC).

A 1V to 4V hall effect sounds like a much more practical proposition than the thing I have here - if you can remember a name or ID to look out for that would be handy (Matt bought this thing expecting it to actually be a pot....)

And thanks for the feedback guys ;^) I'll try to get some more (exceedingly) rough science done on that old battery mwahahaha!

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Jeremy
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Re: Li-poly batteries - incendiary device look out!!

Postby Jeremy » Sun Jan 23, 2011 5:33 pm

Bob,

There's something awry with that Longma throttle, as I have one here and it conforms to the standard 1.2 to 4V output when powered with 5V. All of these Chinese Hall throttles seem to be near-identical in terms of output, the only variation seems to be with the connectors and wire colours (they are usually wired with red, green and black wire, where red = +5V, green = throttle voltage and black = 0V). Sometimes you get variants that are wired with red, blue and black wire, where the blue is the throttle voltage output. They all work on around 5V, although I've seen a few controllers that run the throttle on 4.5V and one that runs it at around 8V. The max safe voltage for the sensor in the throttle is around 12 to 15V I believe.

You'll find the Chinese Hall throttles pretty fragile. I've had several fail for no apparent reason and now only use the Magura pot throttle, which is a hundred times better and more reliable in my view, even though it's a lot more expensive. The failure modes with the Hall throttles range from Hall element failure (pretty common, it's a cheap Chinese knock-off linear sensor), magnets and Hall sensor coming unglued, wires dropping off internally (they are very crappily constructed) and the spring failing, often jamming the throttle wide open. The sensor can be replaced pretty easily with the Honeywell SS496, which fits perfectly. This fixes the sensor failure mode, and probably the crappy internal wiring issue, but leaves you with the mechanical fragility failures as a fairly strong possibility.

The pot in a servo tester can be swapped out and the thing can very easily be driven by the Hall voltage from the throttle, there's a neat thread somewhere on ES showing how it's done. It works pretty well and is a cheap, quick and easy way to get an RC controller running on a bike.

Jeremy

bobc
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Re: Li-poly batteries - incendiary device look out!!

Postby bobc » Mon Jan 24, 2011 11:11 pm

I ordered another twist grip - I did actually dismantle the first one to see what was in it, which might explain if its performance is not as originally designed! I've also ordered one of the $14 8S lipo voltage monitors from hobby city - it has an alarm output which I can use to kill the drive. I'll make my throttle/ESC interface board work right with a properly working throttle before I publish any schematics! I had already bought that very servo tester Jeremy linked & it did the job nicely for initial "bench tests".
By the way (grandma this is how to suck eggs...) it's worth reading the reviews on the product pages on hobby city; while you do that an extra discount offer on the part seems to always flash up - usually worth 5 to 10%
Bob
PS I always appreciate your words of wisdom Jeremy & the time you waste on us strangers - thanks!

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retepsnikrep
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Re: Li-poly batteries - incendiary device look out!!

Postby retepsnikrep » Tue Jan 25, 2011 6:01 am

Agreed there is a bit of urban myth status for lithium fires.

Yes there have been a few examples. But not hundreds every week.

I'm sorely tempted by a pack of nanotech lithium for a new project!
Regards Peter

Two MK1 Honda Insight's. One running 20ah A123 Lithium pack. One 8ah BetterBattery Nimh pack.
One HCH1 Civic Hybrid running 60ah A123 Lithium pack.


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