I really don’t quite know what to make of this .
Interesting. How does it keep capacity without being longer?
Edit. Replace protection with charge circuit?
Is it safety??? 
A micro usb port is 3mm thick. I’m guessing the charge rate isn’t as fast as a standard charger since li-ion charger chips generate more heat at higher charge rates. Not a good idea to have lots of heat right on top of the cell. A more efficient switch mode li-ion charger wouldn’t fit.
I don’t see any mention of it being protected but you might be able to fit it. They seem to make protection chips with integrated mosfets. Saves space vs the separate mosfets that we usually see in protection circuits.
brr does not look good in my eyes.
i have seen cells where you can remove an end part to reveal a USB connector for charging, not Li Ion though.
I prefer a normal charger
I saw one somewhere and it looked like an 18500+charger
A 18500 does not store 3400mAh of energy. If it uses one of those standard charge chips it should be safe and fine. If it is set at 200 (or so) mA charging, the heat produced should be fine too. It will be slooooooow though.
This sounds like a job for (super~~) HKJ. :~~)
I would go for the 18500, too.
I haven’t heard of MecArmy so far - so maybe the 3.4Ah aren’t quite what’s inside?
says its 70mm tall, so it looks like they are adding 5mm for the charge circuit, i would expect fitting issues in certain lights
I think it uses a 18650, the charging system in their lights are fairly slim and the components are on the same side of the pcb as the microusb connector. I see it absolutely possible to take the charging circuit of the PT series light and integrate it in an 18650.
Interesting
Many USB ports struggle to put out more than ~500mA, so expect v-e-r-y s-l-o-w charging. Which means a lot of unmonitored charging will be going on while people are sleeping. NOT a good idea.
Phil
It would be more normal to charge things off a cell phone or tablet usb AC charger than a pc. And 1A is the minimum you would expect for any modern usb AC charger these days. If I was buying one separately I wouldn’t waste time on anything less than 2A with auto coding ports.
The heat that li-ion charging chips generate will likely be the real issue limiting charge rate. Lots of heat right on top of the cell is not such a good idea.