[rdl]

Most modern rechargeable batteries have fairly sophisticated chemistry and thus charge controllers needed -- you don't just dump electricity into them blindly and then take it out. It's not quite rocket surgery, but there's a reasonable level of engineering involved, especially if you want optimal performance with the newer standards (which increase voltage on devices which support it, etc.)

[mrguyorama]

Often times that "reasonable level of engineering" only needs to be slapping a ten cent IC on a board with some manufacturer default application circuit.