I have no schematic of this device and am not familiar with it, so this is just a logical educated guess.
It would seem like an external source would be connected to the 3 pin battery connector.
Black, White, Red top to bottom. Black being GND, Red being Vbus, and White being a sense line.
However, since this charges via a USB port, it would also seem like it would run via power to that port even without a battery.
So, the question is, does it? If so, why go thru the trouble of the buck converter?
If you have connected Black to GND and Red to + and it doesn't work, it may be that the White line needs some voltage to allow it to turn on. That might be a temp sense line.