Ok.. sounds doable, but if I was designing this, I would buy my Lithium battery from a reliable vendor that provides full datasheet of the battery.. wouldn't use a cheap battery and just assume max charging current, charging voltage.. or even capacity. Good thing is that the battery is probably disconnected from the load by default (when charging) and when the user wants to smoke he pushes a button that connects the battery to the load. Bad thing is that the battery is between your teeth when it explodes.
Well that is why he started this thread. And I think this is kind of a good project, but you are right, he must fully understand what he is doing.. it is not particularly hard, but there is a lot of details that needs to be taken care of. Take your time, study, build a prototype, but do not connect it to a battery before thoroughly testing the circuit.