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.
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.)
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.
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.
This is right. But this time that was not the primary reason. From earlier experiences, Badge.Team, started well ahead of time and were on schedule, but things went wrong after some drama with the WHY2025 organization.
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.
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?
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?
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 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.
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.
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.
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)
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.
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.
Readit News
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.
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.
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.
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.
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.
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?
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?
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.
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...
(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.
Deleted Comment
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.