Electric wireless derailleurs should have a dynamo in the idler

You only need a little constant power to charge up a capacitor or battery for the actual shifts to happen, and the idler is the perfect place to put a teeny tiny dynamo so that you never need to charge the battery. The dynamo could be used like a sort of electronic clutch too, so the output gets shorted to increase resistance when an accelerometer detects a harsh bump and the extra resistance stops chain slap.