Tech question about batteries

[puff-puff-pass]

What are the specs for the "common" li-ion battery used in e-cigs?
Charge voltage/rates?
Number of cells?
etc etc....

I am an avid DIYer and am thinking of making my own charger, but need to know the key specs.

 

[krapanj]

These a single cell 3.7v batteries 4.2v fully charged.

 

[puff-puff-pass]

Thanks Peter