Saying a dynamo is constant current doesn't hold value unless you say it "with a resistive load", most USB devices use switch mode regulators to poll the voltage and switch load accordingly - they are not resistive loads.

The Sony M4 Aqua I mentioned earlier, it cycles between 100mA, 500mA, 0.74A and 1.2A, hence the graph of W v kph is not indicative of reality.

I'll stick with the math rather than a drill.

https://en.wikipedia.org/wiki/Electrical_efficiency