How to Charge Rechargeable Batteries Via a Micro USB Port in a Portable USB Charger

I recently made one of the Altoids box USB chargers that have been popping up all over the site and it turned out great, but now I'm looking for a little more of a challenge, so for my next endeavor I'd like to make a portable USB charger which uses rechargeable batteries that can be recharged while still in USB charger through a micro USB input (like how you plug in a phone to charge the internal battery).  I've seen a few projects that use solar panels to achieve a similar result but (perhaps just due to my own limited knowledge) I couldn't figure out how to make it work with a micro USB port.

Also, if possible, I'd love to be able to use the 18650 (EFest IMR) as the rechargeable battery/batteries.  I have a few of them that I use for e-cigarette mods and they hold a good amount of juice so why spend more money on rechargeable batteries when I already have some.  But if using more conventional batteries would make things easier then that's not a problem at all.

I can't wait to hear the community's input on this.  And to those who are the knowledgeable ones in this field, don't feel obligated to say everything entirely in layman terms for my benefit  - what I don't know I will look up; how else are we supposed to learn new things anyway?  Thank you for your time and for any help you can offer!

-John

You can use a single 18650 in conjunction with a boost converter like the minty boost to get a 5V USB output from the 3.7V battery. Then all you need is the charge circuit for a single battery and the 5V from a micro USB connection should be able to charge the battery. Most of the wall chargers for these batteries charge them in pairs but they are in parallel when in the charger. So you should be able to charge a single battery off the circuit. Just a matter of pulling the charger apart and removing the charge circuit from the power supply in there that is taking the wall power and dropping it to the 4.2VDC the circuit requires. Be sure to do some testing and make sure the circuit can handle 5V without damaging anything. I suspect it won't be an issue. Most USB ports will be putting out between 4.7V and 5.2V anyway.