It has a USB input to charge the GB40 and a USB output to charge a mobile phone with the GB40, for example. Could it be that you have mixed up the connections? If that doesn't help, ask for a guarantee and send it back. I have 2 GB20s and 2 GB40s and am very satisfied, but unfortunately I also had to send one back because it wouldn't charge, but it wasn't a problem, I got a new one after two days.