I'm thinking you dont have the E-werk but the USB-werk?
I suspect that the USB-werk is not designed to be used with the cache battery you linked to.
Another potential is checking the cable. Some of the cables provided with the original E-werk had different
pins so although they plugged into some electrical item, they wouldnt charge them. My garmin edge 800 is an example of this. I had to use a specific cable (provided in the E-werks retail box) for my garmin.
Please clarify via the different starbike.com links which E-werk version you have. I'm aware of two different ones. To the best of my knowledge only one is for using with the separate cache battery.
The other is the USB-werk and has no variable voltage/amps.
Original with variable voltage/amps for use with cache battery:
http://www.starbike.com/p/Busch-M%FC...RK-361-3707-en
Cache battery for above/original version:
http://www.starbike.com/p/Busch-M%FC...y-461A-4152-en
New version USB-werk with inbuilt cache battery:
http://www.starbike.com/p/Busch-M%FC...-361BW-5253-en
"The USB-WERK can be connected to a dynamo hub providing 5 V at up to 1 A. An integrated small cache battery ensures that the power remains constant at slow speeds or short stops. That makes the USB-WERK an excellent solution to charge and power USB devices like mobile phones, GPS, MP3 players etc. Comes with a USB cable"