Just wanted to share some thoughts on this cheap USB charger that you probably already noticed. Can be found in many shops, goes for about 3.5-6$, looks interesting but....
Body is made out of ~2,4 mm thick aluminium pipe, only charges 18650 batteries (protected cells are not recommended, will explain why), blue/red indicator, looks like TP4056's cousin.
Advantages: looks nice, cheap.
Flaws: although half of the body is milled out for easy battery management that is still not enough. Battery slides in easley but you will have a hard time getting it out + you have to slide the battery backwards first and then lift it up so, not possible to do it with one hand only. If you have average hands/fingers you will have hard time and find it annoying and if you have larger hands/fingers just forget about this charger.
This can be remedied in some extent if you stick a piece of lace on one side of charger and then first lie down the lace and then insert battery, that way you can just pull the lace to get the battery out but you still have to slide it down /backwards.
Other problem is that the body is actually conductor for negative pole, so every time you are inserting or removing the battery there is a risk of short circuit, you have to push the battery enough down (compress the spring) so that the positive pole would "jump" over the body edge, if you are not careful and battery slips out of your fingers you will get short circuit, ask me how I come to this :)
If you are using unprotected cells you have about 4 mm of space between battery + and charger body and I really don't see how would you safely manipulate with protected 70mm+ long batteries like protected NCR18650B's
I don't have fancy equipment to do charging algorithm charts but even without that I consider this charger a fail, certainly not something that you would give to someone as a present with the flashlight.
Charging circuit is 20mm in diameter and might be interesting solution for custom built flashlights as a "on board" charging solution, you would probably just need to replace USB socket with that round connector that will disconnect the driver when psu is plugged in.
