There are a few flashlights sporting a USB internal charger that are advertised as doubling for emergency power banks. My question is: why only some can do that?
USB to battery can be a cheap and simple linear regulator IC. Doing it as a switched mode regulator is already more complex, expensive and larger (comparable to linear vs buck LED drivers).
For Powerbank modes you need a chip that can be buck in one direction and boost in the other with the same inductor. Those usually cost quite a bit (see my charger project rn, I think 2-3$ for the chip alone in small numbers). There’s not many battery charger chips that can reverse boost and those are all fairly expensive.
In addition, for a simple current sink (USB port only used to charge the light) the negotiations with the connected device is done by 2 simple resistors. Once the light can also reverse boost, it needs to dynamically negotiate it’s role (either current sink or current source), and that needs yet another chip that needs space and costs money. Or you pick a spendy charger as I did that does this and the buck/reverse boost in a single package (max77757) but those are very rare. I only know of 3-4 models from Maxim Integrated that are even capable of this all in one, and those are all 2-4$. Which is a lot for a flashlight driver PCB. Probably more than all the other parts on it together.
These also need a lot more external components than a simple charger, so space constraints start hitting, too.
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Honestly, while this sounds like a nice idea, I can’t help thinking that an actual power-bank is a better solution especially as this can also be used to charge the light.
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It’s actually quite nice, using it a lot on my SP33S. I’m carrying the light in my backpack for travels anyway, and the 26650 makes for an excellent emergency power source. This way I don’t have to carry 2 separate devices.
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So if I connect 2 lights that can each act as a powerbank, do they both get confused as to whether A charges B or B charges A?
Bah.
Unless I’m carrying some Q8-sized light, I don’t want it bleeding off power to something else.
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Depends on the implementation. USB-C is pretty intricate in that regard, dual role devices can advertise themselves as “preferred source” or “preferred sink”, or as “doesn’t matter”. If two identical devices meet, it is usually random who gets to be source and who sink AFAIK.
In the case of smarter devices like cellphones the phones have a selection in the quicksettings dropdown to swap roles if you connect 2 of them.
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At best, it can serve in an emergency capacity and provide some utility.
At worst, it can compromise the fundamental performance of the light, or introduce other issues. The revised TS21 that added the powerbank feature comes to mind.
Of course, one could argue that there is nothing inherently wrong with such a feature, if properly implemented, but the brands we discuss here don’t have a pristine track record in that respect, do they?
Given, that, I fall on the “proper tool for the proper job” side of the fence, not the “try to be jack of all trades” side.
What happened to the TS21? Any detailed reports?
I love the idea of a 21700 light with this feature for things like a roadtrip where I may not need either but it’s nice to know I have them, and can even be charged in the car as I drive if really neccessary. To me it’s the same idea as a mutli-tool; I’m not taking a whole toolbox on holiday but a multi-tool in the glovebox or bag brings a little piece of mind.
It requires a big ass inductor to boost the voltage to 5V. I laugh at their attempt to include such circuitry and PCB real-estate in a light that comes with FET drivers. They could as well build a well regulated driver with all those efforts instead and let the lights do their primary job better. Then I might be interested in buying them.
The original version without powerbank was ok. The newer version with it had some issues detailed below, as well as odd behavior from the switch indicator light.
Technically, it doesn’t have to be a particularly big inductor if it’s only boosting to 5V.
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I have an Acebeam Ec35 Gen ii (In fact I have 2 spares as I love it so much and it’s discontinued) which is 18650 and has powerbank feature (usb-C in and out). This is the only one I have, but I really like this as all my main lights are 18650 (main EDC is Zebralight SC64c LE) as well as a few 18650 Hank lights, so I can just add more capacity to my acebeam powerbank system by bringing more lights – it also justifies having it to other people even during the day ‘Yeah I just have it because it’s also a powerbank’ 
I also have other powerbank solutions (including the Xtar 2*18650 powerbank thing which complements this whole system) but I love that I can just carry this (plus a tiny c-c cable in my pocket) and have some backup power for my phone if needed.
And they have at most 15W, most of them probably less. Compared to boost regulators pushing 20-30W through a LED, that ain’t too bad.
Still, bidirectional DC-DCs aren’t cheap.
I’ve seen them go back and forth until theyre both dead lol
A charges B but dies before B reaches 100%. B sees A is dead and starts charging it but of course B now dies before A reaches 100%. Now A, now partially charged up, sees B is dead and starts charging it again. And so on and so on. Easy to see where this is going.
So you had one that was at 90% and one that was at 20%, you come back and they’re both 0% and 100 degrees lmao
That is so stupid LOL.
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