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.
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.
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.
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)
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.
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.
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.
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.
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.)
Curious to learn more as an outsider if you don't mind elaborating?
Oh, and the entire Team:Warehouse resigned on day 5 of the event.
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.
Two 18650s also sounds like it would be heavy for a badge
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.]
18650 means 18x65mm, but "protected 18650" usually means that cell with electronics stuck on the end, which ends up at 68-70mm long. Nobody calls the result "18680" even though that describes it more precisely.
There isn't a common cell slightly shorter than 18650 to produce protected cells that fit in every holder designed for 18650. The AA-size (but not AA-voltage!) 14500 often does come in protected versions that are exactly 50mm long, based on 14430 cells.
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.
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.
This one has a big colour screen and a full qwerty keyboard. It's not really to be used as a badge. More as a way to get people interested in making something cool with it.
After all, hacker conferences these days are about 50% traditional security and 50% makers.
Lower voltage, of course.
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.
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?
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.
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
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.
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?
Many people also like to have fuel gauging but that isn't required.
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.
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.
bravetraveler•6mo ago
pjc50•6mo ago
arghwhat•6mo ago
(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.
piva00•6mo ago
arghwhat•6mo ago
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.
phire•6mo ago
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.
farhaven•6mo ago
It's likely not voltage because they're connected in parallel.