After a quick google for one of these 2x AA or 2x AAA USB chargers from Walmart, and not finding a satisfactory result, I decided to have a look for myself, and post it. My concern was that I have been using this charger as a backup (in the big black man bag), but never been sure if it has any smarts in it or just a dumb resistor inside of a black box. I see that it does have a network of transistors and other components, so I would guess that the circuit is smart enough to not overcharge. Now I have some trust in it. Said that, no, I will not bother reconstructing a circuit diagram on this occasion -- this result is good enough for me.