The Chinese therapy balls have long been used in Chinese medicine as a way to stimulate acupuncture points in the hand and to loosen stiff joints. As someone who suffers from hand pain on occasion, I think that they’re a great way to improve mobility in your hands and fingers. A product designer by the name of Jiang Qian has combine this hand therapy gadget with a battery charger!
Essentially the balls convert rotational kinetic energy (i.e. movement) into electricity that chargers the batteries internally. As you move the balls around each other, they drive a tiny dynamo internally. You can tell if the AA or AAA battery is charged or not due to the built-in LED light. I just wonder how long it takes to charge up a single battery!?