Fire hazard of WHY2025 badge due to 18650 Li-Ion cells
65 comments
·August 6, 2025eqvinox
unwind
That is mentioned in the article, although could perhaps be more emphasized since it does mean the "obvious" fix is not possible:
Commonly available protected 18650 cells don't fit in the badge's cell holders because they are slightly longer.
eqvinox
I did indeed totally miss that, thanks for pointing it out!
krekr
I’ve worked with the Keystone holders and can confirm that those will hold protected cells, at least some I got from nkon.nl
supakeen
Sure, that means the idea of using unprotected cells was already there when the holders were selected :)
eqvinox
Yeah, I'm just saying, you can't even buy regular protected cells and put them in, because they won't fucking fit. I do think "actual" 18650 protected cells exist, but they would be rare and expensive because you can't build them out of mass manufactured bare 18650 cells (for obvious reasons of where do you put the damn protection circuit.)
supakeen
Ah yea then I misunderstood. That's right you can't easily switch out the cells for protected cells yourself :(
leptons
The Keystone holders are nice but expensive, but they do not fit most protected 18650 cells, and I don't like the PCB mounting options.
I designed my own 3D printed 18650 holder for my project, including a positive battery tab cut-out to prevent reverse battery insertion. I get to decide how big the battery can be, and protected cells are 100% the way to go.
I've never had a problem with a short with the protected cells, and my circuit also cuts off power to the load using a mosfet, if a short ever occurs. It's been working great for years.
giantg2
Funny, I'm also going with a 3D printed 18650 battery module... for a lepton (FLIR) project.
knotimpressed
Braving potentially getting spit roasted to ask:
If I'm making my own 18650 USB C power banks, are there any easy to miss risks? I've got the cells in holders, not welded, but the holders are specc'd above the current I need. The cells are unprotected, but the Aliexpress listing for the power management board says specifically to use unprotected cells, as at 6A draw most protection boards don't do well (dubious). The cells are tested and mechanically protected by a thick enclosure. The only EE work I'm doing is soldering 2 high gauge wires from the holder to the board that's doing everything else. I know Aliexpress isn't a bastion of quality, but the seller has good feedback and I checked over the board to make sure there's at the very least a good counterfeit battery protection IC included.
Currently, the concerns I have are: - the holder relies on good contact to deliver 6A without developing hotspots on the terminals - the board from Aliexpress perhaps should not be trusted
If there's anything else anyone can think of, I'm happy to hear it.
Eduard
it helps if you give us links and model numbers
ShakataGaNai
I love these concept of badges, but almost never are they well executed. Defcon has had TONS of problems with their badges of all types. OpenSauce has tried for the last two years with only middling results.
Don't get me wrong, I'm not trying to say "don't do it" or "These people are stupid". It's just that people underestimate the time and effort required. It's basically bringing a product to market, for 20 to 50k people (depending on the event), in a few months time. But it also needs to be "cool" and "unique" and often "beginner friendly" and extremely cheap. Crazy crazy hard.
Waterluvian
Engineering is the first 80% of work. But productization is the second 80%. I find that the more nerdy a community or product audience, the more the latter suffers.
andrewstuart
Avoid battery fires: design your boards for alkaline batteries.
They’re safer and in many cases just fine for the job - for example a conference badge needs nothing more than alkaline batts.
Also, alkaline batteries are not an expensive nightmare to ship.
burnt-resistor
Rechargeable safe(r) chemistries. Alkaline batteries are insta-e-waste. Like the common form-factors of AA/AAA Ni-MH that are acquirable locally.
rgovostes
Conference badges are basically insta-e-waste too.
em3rgent0rdr
AA/AAA won't fit in a flat conference badge. I've wanted to get coin cell format NiMH, but they only seem to be the more thicker button type (or I don't know the magical term to search for), and I can't seem to find cheap options either. The 40 maH ones seem to be 5mm tall and I can't seem to find many smaller capacity ones that are thinner.
michaelt
In the badge from the article they're using 18650s, so AA batteries would actually have slimmed it down by 3.5mm.
Lower voltage, of course.
bravetraveler
That's a lot of juice for a trinket. Shorted a 'mitigated' battery of these in a charger once, was enough to reconsider vaping entirely
pjc50
Given how much torch you can power off one of these, I'm not clear why you'd need two of them for a badge, since any conceivable use for that much current is going to start heating up the badge PCB fairly quickly. They're usually sized for >1A each, and you can get >10A off them for short periods, which is very lively for a badge pinned to flammable clothing.
arghwhat
Assuming decent cells with low ESR (say, <30mOhm), one such cell will deliver hundreds of amps when shorted, making things a little bit more lively than your estimation. :)
(A few hundred amps isn't a lot for a shorted battery, but these are tiny cells so that's what you'll get.)
Two cells was probably selected for one of: Voltage to avoid boost converters, capacity to avoid having to do extensive power optimization to make it run the whole event, balance to make it hang even off your neck.
farhaven
> Two cells was probably selected for one of: Voltage to avoid boost converters, capacity to avoid having to do extensive power optimization to make it run the whole event, balance to make it hang even off your neck.
It's likely not voltage because they're connected in parallel.
piva00
Quite oversized amounts of power for 2 ESP32s running, last I measured a ESP32 board I have running it would consume some 120-180mA.
null
1970-01-01
It's all cute fun and nerding out until the room is filled with smoke. The art of one upping your hackercon badges is clearly getting out of control. Good job on calling it out now.
ryao
At work, whenever we design hardware that uses Li-Ion cells, I always discuss safety before we even have done the design work so that we build something that is safe. Why did these guys not do that? Did they only learn about videos of li-ion explosions/fires after design this?
By the way, they probably should have used a LiFePo4 chemistry instead. It would not have the same runtime, but it would be much safer in worst case scenarios.
krekr
I've also written up some more findings. They can be found at https://www.krekr.nl/content/why2025-badge-fire-hazard-addit...
bsder
> The WHY2025 badge was designed to be powered by 2 Li-Ion 18650 battery cells connected in parallel.
Wait, what?
I was under the impression that Lithium batteries were really difficult to put in parallel without a LOT of engineering work.
The discharge curve for Lithium batteries is super flat. If you put them in parallel, even a small differential between the two means that one battery will completely discharge simply trying to bring the voltage of the other up to match. This is very different from the discharge curve from alkaline which has a nice slope and the batteries can equalize without burning up very much of their capacity.
These don't look like they're matched in any way. The connection between them doesn't like very big--I suspect a non-trivial voltage drop if one battery tries to empty into the other.
If you need the power, it's much better to put them in series and use a buck converter to bring the final value where you want it.
This seems more like a fundamental engineering flaw rather than a fault in the boards (although, to be fair, the creepage and clearance don't look great).
jacquesm
Paralleling 18650's is relatively easy. You need to match voltage to within a few mV and make sure the connection is really solid (welded) to ensure they stay paired perfectly. Flaky connections, putting cells in series, impact damage, bad chargers etc are the risky bits, a solidly connected pair of 18650's is to a close approximation just as safe as a single cell, but it does have twice the short circuit current so you are going to have to be more careful around them. But at least the casings will be at the same potential.
I've built a 17P10S pack which was a pretty interesting (and scary) effort but it has been working flawlessly for years now with just one inspection of the guts after two years to make sure that nothing was coming loose (it's on an s-pedelec e-bike). In a big pack like that it's the spaces between the alternating blocks of cells and on top where the interconnects are that the real risk lies, besides the fact that the short circuit current of that pack is just shy of a kilo ampere so you really don't want to drop a tool or a piece of interconnect strip on that.
ajsnigrutin
Many devices have parallel lipo cells, from powerbanks to electric cars, nothing special here.
If one cell is weaker, the other provides more current, there is no "one discharging/emptying into the other" during normal work (read below). No real need for any proper matching either, if you only care about capacity (if you care about current, you don't want to get into a situation where any of the cells has to provide more current than designed for and safe.
The only "problematic" part of parallel batteries is making the first connection, where one might be at a much higher voltage than the other. Usually this is mitigated by equalizing voltages (either dis/charging to a fixed voltage, or do a parallel connection through a proper resistor), and after they're safely connected in parallel, it doesn't matter.
On the other hand, two cells, user removable and replacable can cause exactly this issue, where the user removes one, recharges it in an external charger and replaces it (while the other, empty one, still stays inside)... but maybe there's a diode somewhere that prevents reverse currents.
exmadscientist
Li-Ion chemistry is pretty happy with 1S2P configurations. (That doesn't necessarily mean you should do it.) 2S1P is where the fun starts.
eqvinox
It doesn't matter what happens in the 10 to 90% range, if one discharges before the other, it's perfectly fine. It's not like this is an application that needs the combined current capability of both cells. What does matter is that neither cell is overcharged nor deep discharged, and the [dis]charge curve is absolutely not flat in those areas.
stavros
The GP is talking about the case where you plug in two batteries of varying charge levels or health, which I agree is not an amazing thing to do.
eqvinox
That's not obvious from the comment and logically inconsistent with parts of it; if the trace between the cells is small it would act as an auxiliary fuse, and there is that balancing resistor (whose value I can't read because whoever drew that schematic didn't bother repositioning overlapping labels.) I'm also a bit confused about the 2 polyfuses.
That said you're right and I was focusing a bit too much on my reading/interpretation of the GGP post. I'm not sure I've ever seen a 1S2P LiIon configuration with individually user swappable cells. In the 2-cell design I did, I specifically decided to go for 2S1P and have the balancing circuit, to avoid this exact issue. It does have the downside that you need both cells, the WHY design works with only one populated... (which is what I'd recommend doing in any case.)
[ed.: the balancing resistor seems to be 200Ω. The polyfuses are 15mΩ. So I guess it's designed to trip one or both polyfuses if the cells are imbalanced. That's an... "odd"... design.]
rollulus
It’s what I thought as well, but I’m not too much into electronics to hold an opinion. It looks like there’s a balancing resistor between them: https://gitlab.com/why2025/team-badge/Hardware/-/blob/main/C...
bsder
With floating grounds due to those MOSFETS adding 50 milliohms or so (on the order of the internal resistance of the batteries!)!
YIKES!
eqvinox
Putting the protection circuit* on the battery's negative pole is standard best practice (due to NMOS efficiency, and it not being a problem in the slightest), and the 50mΩ actually improves balancing. Please avoid making comments like this based on half knowledge.
[*] I do wish it were an actual full protection circuit. It isn't. Then again a run of the mill protection circuit commonly doesn't cover reversed polarity [between protector and cell], which is rather important for this specific appliation.
pjc50
Is it just me or is that schematic hard to read due to bits of text being on top of each other? Also "LED will burn when battery wrong way round" .. how about fixing this problem which you have acknowledged? What happens to your balancing resistor when you put one battery one way round?
ajb
"LED will burn when battery wrong way round" .
I don't know about the rest of it, but I think this is just an idiosyncratic translation of "LED will light when battery wrong way round" - IE it's a warning LED.
slacktivism123
What's with the mock-security-advisory with logo for the 'vulnerability' (Heartbleed, anyone?)
Why is the important safety advice buried in a bunch of interpersonal drama and administrivia?
Juerd
Author here. I didn't add the logo, but it's a wiki so others can theoretically change things.
I think the logo is cute though, so let's keep it. I think it was made with the WHY2025 logo generator at https://design.why2025.org/
charcircuit
Hopefully, they get sued to clearly set the precedent that this is not acceptable. Just because it may be a devkit, targeted towards knowledgeable individuals, or amateur made that doesn't mean that they should be distributing unsafe electronics like this because it's cheaper than making a safe version.
seszett
This is the Netherlands, they obviously (and luckily IMO) won't get sued when there has not even been any incident and they have a clear advisory for the safety problem.
I don't even know on what grounds could anyone sue them.
burnt-resistor
Why the fuck didn't they just use say 4 safe(r) CR2032s or simply have only +5V USB-C power in?
They could've eliminated most of the risk by simply ripping the 18650 holders off the badges and rely on USB power.
I'm confused that noone is pointing out most protected 18650 cells won't even fit in those holders, since protected cells are generally in the 18690…18700 pseudo size range. That's too long to get into those holders.
Source: the holders are likely Keystone 1042 [https://www.keyelco.com/product.cfm/product_id/918], which I've worked with before. For a protected cell, cf. for example https://imrbatteries.com/products/panasonic-ncr18650b-3350ma... - note 69.41mm length.
[ed.: it's the China equivalent of a Keystone 1042, https://www.lcsc.com/product-detail/C2988620.html - I can't confirm but am 95% confident a protected cell won't fit; if it would, the hold on an unprotected 18650 cell would be quite loose.]