I received a comment on Reddit asking if it was possible to rewire the battery input to the USB of the NTS-1.
Well, I initially looked into doing that, but since the battery output is already connected to the NTS-1 USB power line, it would create a loop. I'm afraid it would require a different battery management system to do that (some battery management systems use the same lines for input or output).
I may be possible by designing a custom PCB to to break the loop and allow the current only in one direction (either charging the battery, or powering up) but that's beyond my current skills. If anyone has ideas on how to achieve that, let me know :)