Unless I am missing something these USB Passthroughs are gutless wonders, a cable, a switch and a stripped out battery housing with connector. I doubt there is even any current limiting going on.
I made my own and put hot glue around the end of the battery where the wire comes out. With all the reports of these things failing I strongly suspected that the cord not being secured was probably the problem... If that is the case, it really is incredibly poor engineering and IMO abuse of customer. Takes so close to nothing to secure the cord with hot glue.
The other side of that is that the wires in the average USB cable is pretty thin and was certainly not meant to be waved around like it would get loose in a passthrough.