Unfortunately the device was already defect when it arrived. At first I thought only the led-display was broken,. as there was still some power in the powerbank to charge with. Unfortunately, when the powerbank was empty, I noticed it didn't't charge.
I've first noticed the company about the broken led display and they offered a 3€ discount. After I've let them know the device doesn't charge I didn't hear anything anymore. It's over a month ago and I am still waiting for a solution.