Nice little project there, however, not sure about the Li-Ion charging scheme thats in place. As shown it will almost certainly damage the battery and in worst case could cause it to fail spectacularly (i.e. fire/explosion).

You can buy single cell charger chips from companies like maxim that would be a lot more robust, and include safety options like internal overcharge timers and support proper sensed dv/di, trickle and top-off type charging. Might be worth a look. (EE that deals with li-Ion & Li-Poly designs)

Could we just make something like this to use on my original NES?

#5, the battery will only contain the “protection” circuitry, which *should* prevent anything catastrophic from happening. I personally would not count on it to withstand sustained usage in this fashion. It is only supposed to be there in case anything goes wrong further up. I have also seen cheap Li-Ion packs omit even the basic safety package (typically just a thermal poly-fuse).

yo that shit is sick what u done big ups.No serious that to me is a masterpiece you should pat them and sell them lol

That is pretty awesome, you could make 10 and sell them as if they were near original controllers.