This general idea is hardly new. There have been patents for gadgets you
attach to your leg, and body movements generate electricity to charge a
battery.

The problem with this patent is that it uses a piezo device. Unfortunately,
they don't generate very much energy. You'd certainly be able to "top off"
the battery, but I don't think you'd be able to bring it back from a heavy
discharge very quickly. Maybe the former is all they care about.