(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.
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.
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.
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).
YIKES!
[*] 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.
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.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.)
I guess I need to do more research on this.
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.
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.
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.]
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.
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.
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.
...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.
The earlier design has been matured into Konsool [2] and is available as Tanmatsu [3].
(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.)
Why is the important safety advice buried in a bunch of interpersonal drama and administrivia?
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/
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.]
Commonly available protected 18650 cells don't fit in the badge's cell holders because they are slightly longer.
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.
They could've eliminated most of the risk by simply ripping the 18650 holders off the badges and rely on USB power.
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.
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mtlynch•15h ago