How does a charging device connect to a SON hub dynamo in addition to a headlight?

Connect charging devices to a hub dynamo in parallel with the headlight, not at the rear light output of a headlight. SON manufacturer recommend to crimp 4.8 mm piggy-back connectors to the feedcable of a charging device. The headlight cable connects to the tongues on the piggy backs. If you connect and disconnect the charging device frequently, the Coaxial Junction Box will be a good choice. (see SON Accessories).