OK so you have attached the new circuit to the +/- of the battery directly....???
So you now have an un-fused circuit which is never a good idea, the fuse's job is to make sure IT melts before the wires carry current at such a level they can heat up and cause issues i.e. fire and other lovely things....
I would not run it unless it was fused - you need to know what the wires are rated to carry, doubt it is much being a USB charger, they most likely will not be very large gauge...
do you have a link to the device you installed????
Regardless don't connect it all up until you know why the 15A fuse did not work, USB devices usally charge at 5V and normally we are talking Milliamps to 1/1.5A i.e. 1000-1500mA current draw
A 15A fuse for this application may well be incorrect, the wires, depending on their gauge will have a specific amperage they can carry and a specific fuse will be needed to protect them from over current situations...
Regardless be careful as i am no electrician but this stuff is important
Anyone around here offer any advice?