I opened up one of the Powerbank packs. It's quite different from the Duracell. The Duracell had a huge number of discrete components, which probably cost a lot. The Powerbank looks like it uses a few stock parts.

Opening it up is easy. The white top is fairly soft plastic, so a small straight screwdriver can gently pry it off. Inside is a tiny board with the connectors, and a larger-than-AA cylindrical LION battery. The wires from the battery are nicely thick. The board has two LEDs: red when charging, blue when powering. One quirk is that it won't do both at the same time, with precedence given to charging, even when it's at full charge.

On the component side, there is one 8 pin chip and two 6 pin chips, a coil and a few other bits. No markings on the chips, but I suspect they are common parts that can be guessed. This is probably the same mass-produced guts inside most similar battery banks. Not bad for $3.