I just took a look at the functions in the link you provided and there is tons advanced functionality in the Texas Instruments PMIC module. That’s really nice to see. It looks like I can even customize the battery charging chips charging voltage and have it work with a different battery chemistry possibly.
So it sounds like you just want to know when the Input power source that feeds the regulator that feeds the processor and uBlox module switches from running on USB Input over to Battery Power.
I think this function should tell you which power input source is currently feeding the buck regulator. They do not show any example code over the right so its hard to tell what the function return info will look like but I would try this to see what the function returns when running on USB vs battery power. If you get 2 different returns then you can use it to notify you when the Electron switches input supplies.
Let us know what you find out. I’m excited to see this level of customization being possible.