Charger for 3.7v mod

[Dj Xy]

I'm working on a mod using a battery from a rn4081 the entire mod will be in a zippo lighter case, with a spring loaded 501 connector, what i need to figure out is how to charge the battery externally as there is no room in the case for a charging circuit.

 

[GRTWHT]

Just a thought off the top of my head: install a jack (1/8" headphone perhaps) and connect to an external charger that way?