Oh yes, I'm a pro at this by now. If you unscrew the flap on the back, the battery is underneath. If you disconnect the battery, i.e. remove the connection contacts, then connect the unit with the mains cable, you will have 10 seconds to switch the unit on using the big round button. This would prove that the battery is bad, but the box is OK.
If you accidentally connect the battery the wrong way round, there is another fuse in the red battery connection cable, which is probably defective.
I had to replace the battery because mine was deeply discharged, plus this fine-wire fuse because I was too stupid to connect the new battery the right way round.
Here is the link for the new battery.
Teufel Rockster Air batteryGood luck.