> 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).
> 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.
> 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.
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.]
That's a good tip for safety conscious visitors because the webshop is in the same country. Probably a good idea to put it on the event wiki. Is the event still going on?
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.
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.
The battery would be less likely to enter thermal runaway and explode with LiFePO4, but this level of short circuit risk is still a significant fire hazard. 3.6V / 0.04 Ohms[0] = 90A.
You are proposing replacing a ~20Wh battery with a 2.8Wh battery. The maximum current draw from button cells is also likely inadequate for the application.
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.
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.
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.
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.
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.
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.
28 comments
[ 2.9 ms ] story [ 61.4 ms ] threadWait, 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).
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.
...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.
Why is the important safety advice buried in a bunch of interpersonal drama and administrivia?
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.]
A friend found a 68.8mm cell that fits; the Keystone holder caps out at 68.88mm. Most protected cells are 69.2mm…69.8mm.
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.
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.
[0] https://lygte-info.dk/review/batteries2012/LFP18650%201500mA...
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.
I don't even know on what grounds could anyone sue them.
> 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.
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.
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.
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.