for the OP, from my meager reading...
Li-Ion chargers range all over the place in how they do what they do. They are not typically setup to be in use while charging. You would need to identify the chip on the board you're thinking of using and scouting Candle Power Forums for chargers that use that chip, or you could buy one from a guy over there cottonpicker i think, who makes little dinky usb chargers for batterys, or ask him if one is possible - he makes basic ones for 18, or with displays for 50, and they terminate properly, and use CC/CV properly, which most cheap ones don't, which is okay if you're just charging a little e-cig battery that when it pops will stain a carpet, but if you're using larger 18650 cells your house might be at risk.
that said, there's folks who have been happy with their little USB charger built into a mod, with a 14500 most recently, cute little green job, looks like a floating glowy PC Board, and there is the option to modify a real Li-Ion charger to have a pig-tail and build a charging adapter into the mod. but again, how use will effect performance as the charger attempts to slowly ramp up voltage as it ramps down amperage I don't know. I've read some chips or schemes used in the cheap chargers (can't remember models) constantly reset themselves as they measure the battery, and then re-start the charging, so that type would probably have no problem being used while charging.
most Li-Ion chargers will attempt to mimic the "best practice" of CC/CV charging (first constant current, or amps, then at the half-way point ramping down the amps as the voltage remains constant, so it finishes getting all of it's amp hours at the same time it reaches it's final voltage). Li-Ion's main charging hurdle is getting the ions to travel back, they're like stuck to the terminal where they end up, and a voltage slightly higher than that of the battery must be applied throughout the charging cycle to get them off of it.
did that have anything to do with your question? there's boards out there in the size you mention that'll charge it, (other than for the above reasons) but not provide enough power to vape on from just the board itself. again, whether i'm providing any usefull info escapes me, hahaha
vaked again
