This summer, we saw the WHY (What Hackers Yearn) event happen in Netherlands, of course, with a badge to match. Many badges these days embrace the QWERTY computer aesthetic, which I’m personally genuinely happy about. This one used 18650 batteries for power, in a dual parallel cell configuration… Oh snap, that’s my favourite LiIon cell in my favourite configuration, too! Surely, nothing bad could happen?
Whoops. That one almost caught me by surprise, I have to shamefully admit. I just genuinely love 18650 cells, in all glory they bring to hardware hacking, and my excitement must’ve blindsided me. They’re the closest possible entity to a “LiIon battery module”, surprisingly easy to find in most corners of this planet, cheap to acquire in large quantities, easy to interface to your projects, and packing a huge amount of power. It’s a perfect cell for many applications I and many other hackers hold dear.
Sadly, the 18650 cells were a bad choice for the WHY badge, for multiple reasons at once. If you’re considering building a 18650-based project, or even a product, let me show you what exactly made these cells a bad fit, and how you might be able to work around those limitations on your own journey. There’s plenty of technical factors, but I will tell you about the social factors, because these create the real dealbreaker here.
Three Thousand Participants
The main social factor can be boiled down to this – a 18650-powered WHY badge can start a fire through being touched by a 5 cent coin, a keychain, or a metal zipper of someone’s jacket. This is not a dealbreaker for an individual hacker who’s conscious of the risk, though it’s certainly an unwise choice. For three thousand participants? You have no chance.
A 18650 cell is like a bigger sister to an AA battery – power at your fingertips, just, you’re playing with heaps more power. You can take a 18650 cell and have it power a small yet nimble robot on wheels, or an ultra powerful flashlight, or a handheld radio packing quite a transmit power punch. You can release its power on accident, too, and that gets nasty quick.
Short-circuiting a 18650 cell is a surprisingly straightforward way to melt metal, and by extent, start a small fire. It’s also not that hard to short-circuit a 18650 cell, especially and specifically unprotected ones. This is a big part of why consumer oriented gadgets use AAs instead of 18650s – it’s perhaps less powerful, sure, but it’s also a significantly less dangerous cell.
The Instructions, They Do Nothing!
WHY sold a little over 3700 tickets. I would not expect 100% attendance, but I’m comfortable saying it must’ve been around three thousand people. Sadly, “three thousand people” is far beyond the point when you can hope to give people handling instructions for something as easy to mishandle as LiIon cells, even for a nominally hacker audience.
Of course, you can try and give people instructions. You can talk to each badge recipient individually, release booklets demonstrating what to do and not to do with a 18650 cell, add silkscreen instructions for a just-in-place reminder, or maybe have them sign a release form, though it’s unlikely that kind of trick would be legal in the EU. Sadly, WHY organizers never came close to doing any of these things. It also wouldn’t really matter if they did. These instructions will always, inevitably be outright ignored by a sizeable percentage of users.
Handling unprotected batteries requires cautiousness and some helper equipment. You can’t hope to transplant the cautiousness, at most you can try and issue the equipment. Which equipment? A small storage cases for the cells (must have when transporting them!), as well as a case for the badge, at the very least; to my knowledge, the WHY didn’t issue either of these stock. An ESD bag doesn’t qualify if it doesn’t permanently cover the badge’s back, because any temporary protection is nullified by a budding hacker getting tired of carrying two 18650 cells on their neck, and throwing the badge into the tent without looking. Where does it land? Hopefully not onto something metal.
You can build a badge or any sort of other device using unprotected 18650s, which expects the end user to handle them, like the WHY badge does, and it will be more or less safe as long as the end user is yourself, with 18650 handling experience that I’m sure is to match. Giving it to a friend, caseless? You can talk to your friend and explain 18650 handling basics to them, sure, but you’re still running some degree of risk. My hunch is, your friend could very well refuse such a gift outright. Giving it to a hundred people? You’re essentially playing with fire at someone else’s house.
Just Why Did That Happen?
Hackaday has traditionally used AA cells for our badges, which has definitely help us mostly avoid any Lithium-related issues. Most other conferences have been using pouch cells, which traditionally come with short-circuit protection and don’t threaten to ignite stuff from contact with a piece of metal. 18650 cells are not even cheaper at scale – they’re nice, sure, I wrote as much, but those nice things are quickly negated by the whole “firestarter” thing.
On the other hand, 18650 cells do work for a hacker or a small team of hackers skilled enough to stay cautious, and it also works well at scale when the cell is permanently encased within the shell, like in most powerbanks and laptops. It fails as soon as you expect people to plug batteries in and out, or carry them separately. Respecting Lithium-Ion batteries means being aware of their shortcomings, and for 18650 cells, that means you should avoid having people manually handle them at scale.
Here’s the kicker about the WHY badge situation. I was confused by the WHY badge switching to 18650 cells this year, away from overcurrent-protected pouch cells, which were used by previous iterations of WHY (MCH, SHA) without an issue in sight. So, I’ve asked around, and what I got from multiple sources is – the 18650 usage decision was pushed top-down, with little regard for physical safety. Sadly, this makes sense – it’s how we saw it implemented, too.
The world need an opensource, fully enclosure for a single 18650 cell and multiples ones to be able to be reused in projects. An opensource USB-PD capable power bank with swappeable 18650 cells will be awesome.
They sell the metal spring tabs by the bag, and it’s trivial to make your own holder using them. The problem with such a design would be that people will try to put in mismatched cells in terms of capacity, nominal voltage, and state of charge, which results in very high balancing currents and risk of catastrophic failure.
So, if you really wanted to do it, you would need to invent an open source BMS which accounts for individual cells being wildly different, and can keep track of capacity and balance the load accordingly. Otherwise it’s caveat emptor – use at your own risk.
Cheapness of silicon means some of the smarts at the cell level. Anything else can be layered onto that.
You don’t know whether people will put in protected or non-protected cells.
USB-PD capable power bank with two swappable 18650 cells are available.
Each cell has an individual BMS.
Despite all the fuss about this (again) I’m starting to get a bit curious about how many of these boards actually went up in flames…
I also wonder why so many of these fun little badges, with their sharp edges and sensitive components, do not have a decent casing or cover to protect the bade from being damaged and the user from being hurt. Sure, I know the answer, it has to do with cost, never mind…
it’ll be the aesthetic of seeing all the electronics for the reason why they’re not in cases.
Yeah, this article seems a little pearl-clutching to be honest.
“For three thousand participants? You have no chance.”
Well, apparently they did have a chance, because the event happened and I’ve found no articles talking about any fires. Certainly if there had been issues, the author here could have referenced them instead of solely talking about what MIGHT happen.
Do these cells need to be respected? Sure.
Was this badge a good idea/design? Maybe not.
But I’d like to see a video of one of these cells bursting into flames from “being touched by a 5 cent coin.”
pretty sure they were talking about the electronics being exposed and there being many vectors for a short on the boards?
How many fires would it take to get the event and organisers sued out of business?
Diisclaimers on tickets aren’t worth the ink they’re printed with.
Still feel comfortable and want to take the chance?
Well I’m not the one who ran the event , but it seems the event organizers were comfortable and willing to take the chance.
I still find it very odd for the author to State an unequivocal “you have no chance” about an event that happened in the past and for which, again, there appears to be no evidence that anything bad actually happened.
Was this a massive stroke of luck, or is the risk perhaps overstated?
Admittedly either or both could be a factor here. But humans as a species are incredibly bad at accurately judging risk, both being too averse as well as too accepting (see: someone who is afraid to fly commercially, yet drives like an absolute demon). My money is that the risk is probably overstated.
This is backed up by the fact that when googling 18650 fire, the vast majority of results are articles like this one talking about what might happen, and YouTube videos showing what happens when you deliberately destroy a battery. The few results I found of actual battery fires happening in the wild we’re all in Homebrew power packs for mass energy storage.
Lithium battery fires are, of course, a thing that happens and are documented. From the few stats I found online, the vast majority are in Mobility devices like scooters. Laptops and cell phones come next, then power tools.
I think a more realistic view is that the actual risk of fire from a conference badge is likely vanishingly low. Is it zero? No. But I’d personally wager the chances of someone tripping on a wayward cable and falling bad enough to warrant a trip to the hospital as being higher than a badge bumping against a zipper causing a fire. But that latter risk won’t be the subject of articles calling out the dangers of loose cords.
Personally, I suspect the majority of mobility packs for ebikes, scooters, wheelchairs and power tools involved 18650 cells, they’re near ubiquitous.
There’s nothing wrong with designing for and minimising risk, hell I’d say it was a necessity, both moral and liability, espe.cially if I were handing out my design to 3000 people I don’t know.
And the risk/liability doesn’t stop when the event ends.
Let’s run some basic risk assessment numbers:
If the chance of something happening is P%, the chance of it not happening is (100-P)%.. 1% chance of failure, 99% chance of non-failure. The chance of having N independent tests all give the same result is the product of the probabilities.. a fair coin will flip heads once with probability 1/2, twice with P=1/4, three times with P=1/8, and N times with P=1/2^N.
So if you have 3000 people and want a 50% chance that no one gets hurt, the chance of a single person not being hurt needs to be the 3000th root of 0.5.
ln(0.5)==-693e-3 and -0.693/3000==-231e-6. e^-231e-6==0.99977
So you want a 99.98% chance of each device being safe to have a coin-flip’s chance of 3000 devices being safe. To get an 85% chance of 3000 devices being safe (two standard deviations), you need an individual chance of 99.95%. To get a 95% chance of overall safety (three standard deviations) you need an individual chance of 99.998%.
The next step of risk assessment is to ask, “what reasonable measures did you take to keep the unwanted result from happening?”
In a situation where a 99.9% chance of individual safety translates to a 95% chance of at least 1 in 3000 getting hurt, “we just kinda hoped for the best” isn’t impressive.
Intelligence is recognizing that unprotected lithium cells are to be handled with care. Wisdom is realizing that even the smartest of us aren’t always careful.
The protection circuits can be annoying because they cause problems when you’re trying to parallel cells, since even a small mismatch can sometimes cause very high balancing currents and the whole setup shuts down or even breaks down – so you have to put current limiting balancing resistors and power-ORing switches on the board to deal with the user slotting in batteries with different voltages. It complicates things, and perhaps this is the reason why so many manufacturers choose to glue their devices shut so the user simply does not have access to the battery in the first place.
The other HUGE problem with parallel cells of any type, but especially high-current ones, happens when you put a fully charged cell in next to a discharged cell.
The only safe way to do it is to have a diode for each cell…
Okay. In hindsight a diode per cell is probably not the only safe way to do it, but it’s what I use.
Diodes lose voltage, so that’s not desirable. There are ideal diode chips that deal with the issue, but they come with current limits and they add to your parts count, circuit area, and cost.
I’m curious about the idea that 18650s are easy to interface with.
I know there’s a huge variety of preferences but to me, keeping a pile of holders around is impractical. Like, i have a pile of holders for AA and AAA and sometimes i want 1S1P and sometimes i want 2S1P and for some reason i have a couple 4S1P, and already that’s enough diversity that i rarely actually have the holder i want on hand. And did i mention, the holders are vulnerable to oxidation in the presence of water (or, for alkalines, the lye they leak at end of life), and some of them are impossible to get the cell out of and others don’t have enough force to maintain contact…do people really do this for 18650s?
For rechargeables, I’ve soldered onto AAs before (ugh). I’ve never done any spot welding but i think that’s the gold standard.
Personally, if i’m gonna say “easy to interface with”, i’m talking about the ubiquitous prismatic lipo cells with factory-attached pigtails (and usually a protection circuit too). Always a little nervous soldering on a live wire but i’ve learned the protocol and it’s easy to get a good result.
The annoying thing about cell protected 18650s is that they’re variable in length because the circuit board takes up space at the end of the cell, so some holders are too tight and others too loose, and when you put the cell inside a holder that is too tight it tends to bend the circuitry and sometimes you get intermittent contact inside the cell…
Even AA NiCd or NiMh cells can start a fire if they short in your pocket. Any loose cell without overcurrent protection should be handled carefully.
BTW, there’s an old Boy Scout trick of starting a fire with a carbon zinc flashlight battery and some steel wool.
Fortunately, steel wool isn’t expected to be around PCB’s like in normal conditions, but it would indeed make some nice sparks of molten metal, more then hot enough to start a fire (if done properly).
Regarding the AA NiCd cells, when everyone was carrying a walkman to school, you know, with tapes, back in the 90’s. We had quite a laugh when in the midst of class a classmate suddenly sprung up from his chair and quickly started to empty one of his pockets. What happened was that the one of the fully charged spare NiCd batteries for his walkman short circuited with his keys, the battery quickly overheated and hurt his leg. Not badly though, but just enough to teach him not to carry his batteries and keys in the same pocket.
When i was queuing to get one of those badges, some guys were cooking croque-monsieur on a big camping battery installed in a rolling trolley :-)
I have one of those, searching to use it as an SSH terminal instead of playing Doom…
I would honestly like some data on why dual 18650 were required at all.
Many of the problems would be fixed by protecting the cell, which could probably be accomplished on a single cell (or two side by side) using a simple snap on cover for the batteries, and designing the power circuit with protection for the battery terminal. Something as simple as a cover for the positive terminal and a poly fuse to limit the current to around one amp or two would probably suffice. Even batteries split as these are you can use diodes for possibly mis-matched cells and something as simple as a plate or plates made of cut acrylic would make a back cover.
If hackers want two 18650 they can work out that on their own.
Best option would probably be using a cheap USB battery bank. They can be sourced very inexpensively and are a whole power solution, with protection, case and charging. Also users can simply use their own bank if desired.
Valid concern with Lithium. Ultimately distributing unprotected cells (both electrically and physically) is not a good idea, in fact a bad one. And it’s a matter of when a problem arises.
To those using the logic “nothing bad happened, it must be Ok”. That’s the same logic that resulted in deaths from ice impacts on space shuttle tiles. Ice had impacted the tiles without major problems for years. Eventually the tiles were damaged enough that the shuttle burnt up on re-entry.
A simple first-line solution to the problem is to spray the bottom of the board with PCB varnish, which will coat the exposed pins and at least reduce the chance of a dead short.
How do you short an 18650 with a coin? Don’t you have to remove or cut the shrink sleeve first?
When it’s mounted to an open PCB.
i think it usually requires several coins
Yah, most things in my house, that could catch fire, are well enclosed, packaged in non flammable materials.
The laptops and phones are probably the highest risk items, already a step down from gas consuming appliances or electrical appliances.
But, I have considered a PI NAS with it’s own homegrown UPS. Looking at this article, that is probably not a great idea. Battery in a light weight 3d printed case. I would consider a powerbank, but problem is that they don’t function like a UPS.
It’s hard to make something safe, unless you test it. Who would set a one-of on fire?
With some form of BMS managing the current too and from the UPS what would the potential issue be? As I figure you’d need that in any case for it to be a UPS in the first place. And you’re unlikely to be tossing a bare PCB around in a tent for this use case as cautioned against.
Aside from the inherent risk of Lithium based chemistry; and if you’ve got a phone I figure you’ve accepted the risks of that size of battery in your life.
Also, if they put in protected cells, what is the protection level and thresholds? Is it all the same or different? Is it nearly the same, but still different?
You run into a cascade failure situation where one cell tripping its safety will transfer the load to the others, which then see over-current condition and trip as well. Some of them might reset after a moment, others might not until you remove the load, depending on how your circuit behaves, but the end result may be pulsating power output that may or may not damage your circuit and load.
This is why, when someone makes a battery with parallel cells, they might want to choose to add the protection circuit at the battery level, not the cell level, and balance the load passively using low value resistors or nothing at all and relying on the cells being matched. This cannot be done if the user has the power to swap out cells arbitrarily.
This was in reply to Ostracus in the first comments to Nath, but the comment system glitched out.