3.7V 18650 battery

[HellasTechn]

Ok the second link is a charge controller that terminates charge at 4.2 volts. Then you are good to go !

 

[Kenneth Tan]

I use these charge controllers all the time. Great for charging 1-2 18650 in parallel. Then choose any buck-boost converter that is needed.

I've bought 10 of them recently @ €0,50 a piece.