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.
[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.]
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.)
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.
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.
It's worth noting that this kind of thing works great at a smaller scale. Adafruit has been marketing bare PCBs for sew-on applications for years.
On an even smaller scale: I contributed to a beginner soldering class by designing this simple board shaped like the high school's logo: https://postimg.cc/ftwtqHFn (for the record, shorts across the contacts in the exposed metal area are harmless. The transistor never saturates in this circuit)
The key to Adafruit's arduinos-but-for-cosplay, and my keychain photocell thingy is that neither demands two(!) 18650s worth of power. There just isn't a significant hazard in the first place.
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.
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.
As an electronics designer myself, I would find it hard to trust the AliExpress board.
Without the design files, running a failure modes and effects analysis on the board is difficult.
There's also no guarantee that each board you get is built identically. Some parts or the whole design could be changed between orders.
If I was designing a power bank board professionally, I'd be putting it through the ringer - environmental, mechanical, component level short circuit, load short circuit, load power injection, input over-voltage, input transient, RFI/EMI susceptibility, etc. Do you trust that all that has been done on a board that is representative of what you've received?
Definitely agreed on the lack of consistency between orders and even between boards in the same order. I’m doing very low volume for myself and not giving anyone else the banks, so I’m more than happy to check over all the boards for visually obvious issues with a loupe.
As another commenter said, is there anything beyond short circuit/overcurrent for the load side, and undervoltage/overcurrent protection on the cell side that’s crucial for a non professional bank? I’m happy to pop a few boards testing them myself.
There's a difference between acceptable standards for a truly professional product that you'd have to certify and hobbyist stuff.
Beyond short circuit/overcurrent, overtemp, over voltage, under voltage protections, what else would be necessary to ensure safe usage of the cells from an electrical perspective? Ie. What additional protections would a batter management circuit need to be safe in normal circumstances?
There is a subreddit dedicated to 18650s where people make power banks and the like, https://old.reddit.com/r/18650masterrace/ . Off the top of my head I remember reading about making sure you use the same set of cells together so you don't get unwanted current flow from highly-charged cell to low-charge cell, but I'm sure there's other details too
Absolutely, I’ve browsed that sub a bunch but frankly I trust the opinions here more.
I’m using matched reclaimed cells that I’ve tested to make sure they’re still healthy.
Well, there’s only one real part (the cells are interchangeable, and reclaimed)
https://a.aliexpress.com/_mKNNnGZ
I’ve gone with the purple C and U model.
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.
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.
> 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.
That is highly dependent on the exact ESP32 model and current power state. They can use in excess of 500mA with peaks of 800mA, and can easily use >250mA constant at 3.3V. Some draw much more than that.
A board with a few bits and bobs on and a single 18650 cell might only last, say, 8 hours on a charge.
Now, a well optimized board with a low-power ESP32 and proper use of sleep states would make that number go from 8 hours to over a week, but that does take a lot of extra effort and may not be worth it over just slapping on another cell.
2 ESPs and a 4” color LCD screen (and a keyboard).
Depending on how bright the backlight was, that could eat through battery. And if they were using the wifi for any active communication, that increases power too.
I suspect they wanted it to last the entire weekend with the display always on. The original design probably only had one cell (maybe even smaller battery with built-in protection) and they hurriedly switched to two 18650 cells at the last moment.
They probably went with parallel because that seemed easier, no need to switch to another voltage regulator and charge controller.
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.
citation needed. From mining over production to recycling, NiMH batteries are ecologically inferior to alkaline batteries. A breakeven and superior performance may only set in after many recharge uses which NiMHs may never reach (ageing, rare usage)
It's a bit odd to declare "citation needed" and then claim things like "rare usage" which just so happen to suit your argument, while ignoring things like, say, the fact that NiMH batteries mean batteries are only shipped to the end-user once.
I use NiMH batteries in all my thermostats, two scales, etc. Bought them 10 years ago or so. The thermostats get charged every few months and the scales every few weeks.
I think ~12 or so NiMH batteries have replaced, by this point, by rough back-of-envelope-math, thousands of of alkaline batteries.
Did it occur to you that probably one of the most energy-intensive parts of a AA battery's life is its transportation from factory to user? Which NiMH batteries only have...once? And most of that transportation is powered by non-renewable fuels, etc.
A quick check on GPT suggests that shipping the thing via ocean freight is going to be comfortably less than 1% of the carbon emissions of manufacturing. Batteries are really tiny, so they ship well, and they are really complex, so they are more difficult to manufacture than say a simple plastic toy.
I'm glad you are making use of your li-ion batteries, I'd love to see aggregate data on that. I know in my own personal life, rechargeable AA batteries usually get lost or forgotten before their third recharge for me. Climate wise, I'm probably net negative overall on my rechargeables.
But it's also kinda not the right thing to focus on for climate. Driving 50 miles in a gas car will cause a greater climate delta than manufacturing a battery. Eating 12 ounces of beef (300g) causes more emissions than manufacturing and shipping a battery. One international flight can be equivalent to several hundred batteries, etc
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.
OK, but my point is more general than reproducing the badge from the article. I was rather pointing out the lack of easily-purchasable NiMH (or other "rechargeable safe(r) chemistries") batteries that could serve as a replacement for a very flat coin cell. For instance, CR2032 (3.2 mm tall), CR2025 (2.5mm tall), and CR1616 (1.6mm tall) lithiums are easily-purchasable, and it would be nice if NiMH batteries were purchasable with those heights.
The fact that NiMH is 1.2V is another thing to consider cause their heights would add stacked, though it isn't too much of an issue cause they can be placed flat side-by-side, or could use one of those ultra-low voltage MSP430s that can run off 0.9 V to 1.5 V.
For the type of applications I imagine, I wouldn't need more than 1mA of current...just enough to run an attiny-like microcontroller and blink a LED on occasion before going back to lowpower mode. It seems NiMH just doesn't come in those small sizes. I don't know much about ML, I'll have to look into if ML and LiR in the smallest coin size are safe enough. Actually now that I've thought about it more I might be able to just use a small supercapacitor for around an hour if I'm really careful with energy consumption.
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.
Most if not all holders I worked with simply can't hold protected cells, which I find shortsighted and stupid as making receptacles and spring contacts that can hold both would be dead easy, but I guess saving fractions of pennies with less plastic, metal and weight is the priority.
Huh? A little CR2032 or similar cell has minimal short circuit current. A LiFePO4 18650 may not set itself on fire if shorted, but it could still set the rest of the badge or your shirt on fire.
> 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).
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.
> 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.
These requirements are already not easy, and there are still plenty of things to consider for using LiPos in parallel (e.g. identical health, preferably batteries from same batch, to increase chance they age identically)
OP exaggerates the difficulty. You can trivially parallel the vast majority of lithium batteries so long as their voltage is reasonably close (I personally wouldn't fuss much over a 100mv difference, or even more in most cases, unless it's a massive battery or a power cell capable of delivering and accepting very high currents - charging most cells will often involve raising the voltage 200-300mv during the constant current phase, so you can safely parallel with a difference like that)
You can match up pretty different batteries in parallel as well. One will take more load etc, but this is not usually a problem. It's not ideal, but I think people often exaggerate the dangers.
Series is much more problematic, since most balancing circuits have very limited capacity to balance mismatched batteries.
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.
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.
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.]
> These don't look like they're matched in any way.
How would you tell?
The problem you're describing is real, but it's only when installing the batteries. And you can avoid it by only inserting batteries that are both empty or both full.
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.
Look at the reference circuits, it's a pair of antiserial NMOS on the negative pole.
(Those 2 protection circuits are at the opposite ends of complexity & features)
To be clear, using 2 PMOS on the positive pole is also quite common, my choice of words with "standard best practice" might be a bit misleading.
> use bus bars to minimize wiring resistance.
Those come after the protection circuit, there should always be 2 MOSFETs in series with the individual Li-Ion cell in a design like this (specifically: user swappable cell).
(Protecting paralleled cells together is kinda nonsensical because you also want to protect them from each other, I don't think I've ever seen a 2P combined protection circuit.)
> Those datasheets show creating a series pack/cell.
You seem to only have looked at the TI one, the Diodes one is for a single cell.
& if the cells are "permanently" connected in a pack, you wouldn't have individual cell protection and just have them properly balanced before connecting them in factory.
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?
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.
> Is it just me or is that schematic hard to read due to bits of text being on top of each other?
This is extremely common in products that are open-source community adjacent. I assume it's some sort of stylistic choice as almost none of these labels are in their default positions and many of the default text sizes have been changed, the designer has put in additional work to make this less readable.
The purple text comes from additional fields added to the symbols (specifically these look like LCSC part numbers). When adding properties there's a checkbox to choose if they're visible on the schematic or not and normally you'd leave in unchecked. Again in this case I can only assume it's some kind of stylistic choice that I don't understand as it's a common thing to see.
Repost from a previous comment of mine, that sort of applies here:
> I get nervous when I see videos of people buying random Li-Ion/Po-battery powered crap from Teemu etc.
> My personal policy for buying anything with such a battery: the seller must have a meaningful presence in my country, selling at least like $10M/year.
I.e. they need to have a large enough exposure to handle a catastrophic house fire if something happens due to e.g. a bad design. I figure that at around $10M/year they start caring, even if they are psychopaths or incompetent.
> The WHY2025 badge was designed to be powered by 2 Li-Ion 18650 battery cells connected in parallel. The cells provided to visitors are of the "unprotected" kind
...I am going to put on my "client-facing consultant" hat for a moment, which means skipping the expletives, and just say that not only is this a Very Bad Design, it is such a Very Bad Design that someone should really have noticed this and not let it happen.
An earlier design by Badge.Team [1] did not use these cells and Badge.Team (no longer associated with WHY2025) strongly advised against the use of these cells.
The earlier design has been matured into Konsool [2] and is available as Tanmatsu [3].
WHY2025 orga has been a shitshow on the social & interpersonal level. The badge team was one of the casualties. (And triggers, from what I've heard they weren't paragons either, but that's even more hearsay.)
(Source: I'm in c3noc/Internetmanufaktur, though not attending WHY. TBH I saw the shitshow coming and decided I don't need it in my life.)
But surely two 18650s are just overkill. I might understand switching from the recommended LiPo cell to a single 18650 for cost cutting reasons (even though that's still probably a bad idea safety-wise), but why two?!
I have a smartphone prototype board right next to me which can be powered entirely from a single (protected!) 18650 cell, and let me tell you that its SoC isn't exactly power efficient. It really seems baffling.
I have a bunch of camp badges there too and while some are really fun boards to play with I wouldn't trade safety for longer battery life with any of them.
Hah I also have a smartphone prototype here powered by a single 18650 and I've already seen one of them turn into a smoke machine because the holder has protruding pins and doesn't fit most protected cells...
Two 18650s also sounds like it would be heavy for a badge
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.
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.
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.]
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.
FWIW, I have battery holders that deathgrip unprotected cells, so even finding a proper holder for those is a crapshoot.
I did indeed totally miss that, thanks for pointing it out!
Sure, that means the idea of using unprotected cells was already there when the holders were selected :)
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.)
Ah yea then I misunderstood. That's right you can't easily switch out the cells for protected cells yourself :(
I’ve worked with the Keystone holders and can confirm that those will hold protected cells, at least some I got from nkon.nl
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.
Funny, I'm also going with a 3D printed 18650 battery module... for a lepton (FLIR) project.
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.
It's worth noting that this kind of thing works great at a smaller scale. Adafruit has been marketing bare PCBs for sew-on applications for years.
On an even smaller scale: I contributed to a beginner soldering class by designing this simple board shaped like the high school's logo: https://postimg.cc/ftwtqHFn (for the record, shorts across the contacts in the exposed metal area are harmless. The transistor never saturates in this circuit)
The key to Adafruit's arduinos-but-for-cosplay, and my keychain photocell thingy is that neither demands two(!) 18650s worth of power. There just isn't a significant hazard in the first place.
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.
It's almost fitting, isn't it? Given what each of these conferences are all about.
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.
As an electronics designer myself, I would find it hard to trust the AliExpress board.
Without the design files, running a failure modes and effects analysis on the board is difficult.
There's also no guarantee that each board you get is built identically. Some parts or the whole design could be changed between orders.
If I was designing a power bank board professionally, I'd be putting it through the ringer - environmental, mechanical, component level short circuit, load short circuit, load power injection, input over-voltage, input transient, RFI/EMI susceptibility, etc. Do you trust that all that has been done on a board that is representative of what you've received?
Definitely agreed on the lack of consistency between orders and even between boards in the same order. I’m doing very low volume for myself and not giving anyone else the banks, so I’m more than happy to check over all the boards for visually obvious issues with a loupe.
As another commenter said, is there anything beyond short circuit/overcurrent for the load side, and undervoltage/overcurrent protection on the cell side that’s crucial for a non professional bank? I’m happy to pop a few boards testing them myself.
There's a difference between acceptable standards for a truly professional product that you'd have to certify and hobbyist stuff.
Beyond short circuit/overcurrent, overtemp, over voltage, under voltage protections, what else would be necessary to ensure safe usage of the cells from an electrical perspective? Ie. What additional protections would a batter management circuit need to be safe in normal circumstances?
There is a subreddit dedicated to 18650s where people make power banks and the like, https://old.reddit.com/r/18650masterrace/ . Off the top of my head I remember reading about making sure you use the same set of cells together so you don't get unwanted current flow from highly-charged cell to low-charge cell, but I'm sure there's other details too
Absolutely, I’ve browsed that sub a bunch but frankly I trust the opinions here more. I’m using matched reclaimed cells that I’ve tested to make sure they’re still healthy.
it helps if you give us links and model numbers
Well, there’s only one real part (the cells are interchangeable, and reclaimed) https://a.aliexpress.com/_mKNNnGZ I’ve gone with the purple C and U model.
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
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.
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.
> 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.
Quite oversized amounts of power for 2 ESP32s running, last I measured a ESP32 board I have running it would consume some 120-180mA.
That is highly dependent on the exact ESP32 model and current power state. They can use in excess of 500mA with peaks of 800mA, and can easily use >250mA constant at 3.3V. Some draw much more than that.
A board with a few bits and bobs on and a single 18650 cell might only last, say, 8 hours on a charge.
Now, a well optimized board with a low-power ESP32 and proper use of sleep states would make that number go from 8 hours to over a week, but that does take a lot of extra effort and may not be worth it over just slapping on another cell.
2 ESPs and a 4” color LCD screen (and a keyboard).
Depending on how bright the backlight was, that could eat through battery. And if they were using the wifi for any active communication, that increases power too.
I suspect they wanted it to last the entire weekend with the display always on. The original design probably only had one cell (maybe even smaller battery with built-in protection) and they hurriedly switched to two 18650 cells at the last moment.
They probably went with parallel because that seemed easier, no need to switch to another voltage regulator and charge controller.
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.
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.
Rechargeable safe(r) chemistries. Alkaline batteries are insta-e-waste. Like the common form-factors of AA/AAA Ni-MH that are acquirable locally.
Conference badges are basically insta-e-waste too.
Mercury free alkaline batteries are not e-waste, even if they are often handled as such for simplicity (battery bad mmmkey?).
citation needed. From mining over production to recycling, NiMH batteries are ecologically inferior to alkaline batteries. A breakeven and superior performance may only set in after many recharge uses which NiMHs may never reach (ageing, rare usage)
It's a bit odd to declare "citation needed" and then claim things like "rare usage" which just so happen to suit your argument, while ignoring things like, say, the fact that NiMH batteries mean batteries are only shipped to the end-user once.
I use NiMH batteries in all my thermostats, two scales, etc. Bought them 10 years ago or so. The thermostats get charged every few months and the scales every few weeks.
I think ~12 or so NiMH batteries have replaced, by this point, by rough back-of-envelope-math, thousands of of alkaline batteries.
Did it occur to you that probably one of the most energy-intensive parts of a AA battery's life is its transportation from factory to user? Which NiMH batteries only have...once? And most of that transportation is powered by non-renewable fuels, etc.
A quick check on GPT suggests that shipping the thing via ocean freight is going to be comfortably less than 1% of the carbon emissions of manufacturing. Batteries are really tiny, so they ship well, and they are really complex, so they are more difficult to manufacture than say a simple plastic toy.
I'm glad you are making use of your li-ion batteries, I'd love to see aggregate data on that. I know in my own personal life, rechargeable AA batteries usually get lost or forgotten before their third recharge for me. Climate wise, I'm probably net negative overall on my rechargeables.
But it's also kinda not the right thing to focus on for climate. Driving 50 miles in a gas car will cause a greater climate delta than manufacturing a battery. Eating 12 ounces of beef (300g) causes more emissions than manufacturing and shipping a battery. One international flight can be equivalent to several hundred batteries, etc
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.
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.
OK, but my point is more general than reproducing the badge from the article. I was rather pointing out the lack of easily-purchasable NiMH (or other "rechargeable safe(r) chemistries") batteries that could serve as a replacement for a very flat coin cell. For instance, CR2032 (3.2 mm tall), CR2025 (2.5mm tall), and CR1616 (1.6mm tall) lithiums are easily-purchasable, and it would be nice if NiMH batteries were purchasable with those heights.
The fact that NiMH is 1.2V is another thing to consider cause their heights would add stacked, though it isn't too much of an issue cause they can be placed flat side-by-side, or could use one of those ultra-low voltage MSP430s that can run off 0.9 V to 1.5 V.
Is a lithium ion button cell (LiR) big enough to pose a danger? I can't really find any tests.
Wikipedia claims several rechargeable chemistries for button cells including NiMH but most of the seem to barely exist.
Do you need more current than ML can provide?
For the type of applications I imagine, I wouldn't need more than 1mA of current...just enough to run an attiny-like microcontroller and blink a LED on occasion before going back to lowpower mode. It seems NiMH just doesn't come in those small sizes. I don't know much about ML, I'll have to look into if ML and LiR in the smallest coin size are safe enough. Actually now that I've thought about it more I might be able to just use a small supercapacitor for around an hour if I'm really careful with energy consumption.
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.
Most if not all holders I worked with simply can't hold protected cells, which I find shortsighted and stupid as making receptacles and spring contacts that can hold both would be dead easy, but I guess saving fractions of pennies with less plastic, metal and weight is the priority.
I've also written up some more findings. They can be found at https://www.krekr.nl/content/why2025-badge-fire-hazard-addit...
Lithium is the wrong chemistry for this novelty throwaway thing. Risks of fire all along the supply and disposal chain.
They could just use a few watch cells. LEDs use no power at all, so lithium not required; and watch cells aren’t hazardous when shorted.
Even the form factor of 18650 seems wrong for this, they would be so bulky compared to watch cells or AAA cells.
Watch cells are also usually lithium. I'd have just gone with LiFePO4 18650 cells.
That doesn’t solve the issue because the fire risk is due to short circuit. Any battery that is Li-based has too low IR to self limit.
Huh? A little CR2032 or similar cell has minimal short circuit current. A LiFePO4 18650 may not set itself on fire if shorted, but it could still set the rest of the badge or your shirt on fire.
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.
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?
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/
> 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).
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.
> 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.
These requirements are already not easy, and there are still plenty of things to consider for using LiPos in parallel (e.g. identical health, preferably batteries from same batch, to increase chance they age identically)
OP exaggerates the difficulty. You can trivially parallel the vast majority of lithium batteries so long as their voltage is reasonably close (I personally wouldn't fuss much over a 100mv difference, or even more in most cases, unless it's a massive battery or a power cell capable of delivering and accepting very high currents - charging most cells will often involve raising the voltage 200-300mv during the constant current phase, so you can safely parallel with a difference like that)
You can match up pretty different batteries in parallel as well. One will take more load etc, but this is not usually a problem. It's not ideal, but I think people often exaggerate the dangers.
Series is much more problematic, since most balancing circuits have very limited capacity to balance mismatched batteries.
> These requirements are already not easy,
Get them reasonably close, then leave then connected by a moderate value resistor for hours. Then you're within mV.
If you're just concerned with getting 95% capacity from each battery, "close" is good enough for the rest.
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.
Li-Ion chemistry is pretty happy with 1S2P configurations. (That doesn't necessarily mean you should do it.) 2S1P is where the fun starts.
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.
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.
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.]
> That's not obvious from the comment and logically inconsistent with parts of it
I agree, but personally I decided to go with the most charitable interpretation, and that's the one that made the most sense to me.
> These don't look like they're matched in any way.
How would you tell?
The problem you're describing is real, but it's only when installing the batteries. And you can avoid it by only inserting batteries that are both empty or both full.
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...
With floating grounds due to those MOSFETS adding 50 milliohms or so (on the order of the internal resistance of the batteries!)!
YIKES!
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.
> Putting the protection circuit* on the battery's negative pole is standard best practice
Pointer? Especially since LiPol paralleling seems to want to use bus bars to minimize wiring resistance.
Admittedly my experience is all about avoiding parallel LiPol batteries ...
https://www.ti.com/lit/gpn/bq77908a
https://www.diodes.com/assets/Datasheets/products_inactive_d...
Look at the reference circuits, it's a pair of antiserial NMOS on the negative pole.
(Those 2 protection circuits are at the opposite ends of complexity & features)
To be clear, using 2 PMOS on the positive pole is also quite common, my choice of words with "standard best practice" might be a bit misleading.
> use bus bars to minimize wiring resistance.
Those come after the protection circuit, there should always be 2 MOSFETs in series with the individual Li-Ion cell in a design like this (specifically: user swappable cell).
(Protecting paralleled cells together is kinda nonsensical because you also want to protect them from each other, I don't think I've ever seen a 2P combined protection circuit.)
Those datasheets show creating a series pack/cell. They don't show the circuitry to then parallel the packs together.
I guess I need to do more research on this.
> Those datasheets show creating a series pack/cell.
You seem to only have looked at the TI one, the Diodes one is for a single cell.
& if the cells are "permanently" connected in a pack, you wouldn't have individual cell protection and just have them properly balanced before connecting them in factory.
> parallel the packs together
You parallel cells, not packs.
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?
"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.
> Is it just me or is that schematic hard to read due to bits of text being on top of each other?
This is extremely common in products that are open-source community adjacent. I assume it's some sort of stylistic choice as almost none of these labels are in their default positions and many of the default text sizes have been changed, the designer has put in additional work to make this less readable.
The purple text comes from additional fields added to the symbols (specifically these look like LCSC part numbers). When adding properties there's a checkbox to choose if they're visible on the schematic or not and normally you'd leave in unchecked. Again in this case I can only assume it's some kind of stylistic choice that I don't understand as it's a common thing to see.
fireworks in the badge? take that defcon!
Repost from a previous comment of mine, that sort of applies here:
> I get nervous when I see videos of people buying random Li-Ion/Po-battery powered crap from Teemu etc.
> My personal policy for buying anything with such a battery: the seller must have a meaningful presence in my country, selling at least like $10M/year.
I.e. they need to have a large enough exposure to handle a catastrophic house fire if something happens due to e.g. a bad design. I figure that at around $10M/year they start caring, even if they are psychopaths or incompetent.
> The WHY2025 badge was designed to be powered by 2 Li-Ion 18650 battery cells connected in parallel. The cells provided to visitors are of the "unprotected" kind
...I am going to put on my "client-facing consultant" hat for a moment, which means skipping the expletives, and just say that not only is this a Very Bad Design, it is such a Very Bad Design that someone should really have noticed this and not let it happen.
Because this really is a Startlingly Bad Idea.
An earlier design by Badge.Team [1] did not use these cells and Badge.Team (no longer associated with WHY2025) strongly advised against the use of these cells.
The earlier design has been matured into Konsool [2] and is available as Tanmatsu [3].
[1] https://badge.team/
[2] https://badge.team/docs/badges/konsool/
[3] https://nicolaielectronics.nl/tanmatsu/
Why was that advice ignored?
WHY2025 orga has been a shitshow on the social & interpersonal level. The badge team was one of the casualties. (And triggers, from what I've heard they weren't paragons either, but that's even more hearsay.)
(Source: I'm in c3noc/Internetmanufaktur, though not attending WHY. TBH I saw the shitshow coming and decided I don't need it in my life.)
"WHY2025 orga has been a shitshow on the social & interpersonal level."
Curious to learn more as an outsider if you don't mind elaborating?
Often it is the case that safety concerns are overruled by less technical savvy managers for some lesser critical reason.
But surely two 18650s are just overkill. I might understand switching from the recommended LiPo cell to a single 18650 for cost cutting reasons (even though that's still probably a bad idea safety-wise), but why two?!
I have a smartphone prototype board right next to me which can be powered entirely from a single (protected!) 18650 cell, and let me tell you that its SoC isn't exactly power efficient. It really seems baffling.
I have a bunch of camp badges there too and while some are really fun boards to play with I wouldn't trade safety for longer battery life with any of them.
A single (removable) 2000mAh lipo cell, on the low end of capacity for 18650s these days, was a very common power source for smartphones in the 2010s.
Hah I also have a smartphone prototype here powered by a single 18650 and I've already seen one of them turn into a smoke machine because the holder has protruding pins and doesn't fit most protected cells...
Two 18650s also sounds like it would be heavy for a badge
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.
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.