Today I received the JaWeTec USB charger for dynamo hubs as like SON or Shimano DH-3D71 or Alfine or similar. This is a small box with an ordinary USB type A connector which provides 5V of direct current and 500mA, which should be sufficient to feed any USB charged devices according to USB specification. It is protected against overvoltage up to 50km/h as to manufacturers documentation and may switch off automatically at extreme voltage or heat. The little box has got 50g in weight approximately and can easily be removed from bicycle by unplugging its connector in the dynamo cable. The box unfortunately is not water resistant. I will attach it at downside of my Medion PNA and put the complete unit into some transparent plastic bag.
The chargers power supply cables had to be soldered to the original cables of my edelux headlight. I opened the protective coating around the dynamo connectors, made a soldering joint for each of the two wires and put new shrink hose around each connector. The charger cable runs along the speedo cable up to the handlebar where it can be connected to the box which will be installed together with the navigation device is mounted.
Update: unfortunately the Jawetec charger is no longer available. There are alternative products available, however.