Capsule. Can I measure Vin?

(Sorry. I was too too tired when I wrote this. My BRAIN meant Vin. my FINGERS typed Vext. Please accept my apologies. I will push the keyboard away when I am tired next time)
Is it possible to measure Vin on a cubecell capsule?

My motivation is to use a large, cheap battery. I have capsules available, and this new application has no space limitations.
Instead of poking about with a jeweller’s screwdriver and soldering iron for a solar panel and LiPo battery, I can plug a large, dumb, disposable 6v battery into Vin on the 20pin connector.
But, can I measure it? I’d like it to alert me (in five years?) that it is going flat.

Hmmm, whether or not it’s possible, and I don’t think it would be, I don’t think applying any input voltage, let alone something more than 3.3V, would be a good idea. Perhaps refer to the relevant part of the schematic:

This schematic would appear to support my understanding that Vext is a voltage output, not an input—it is simply fed through a MOSFET from the internally regulated 3.3V source.

According to the above schematic, there are several points at which you might feed in an [appropriate] external voltage, but I’m not sure you’re going to be able to measure any of them ‘internally’, except that supplied through the battery input header—that’s where most Heltec products expect you to be supplying battery power and, accordingly, that’s where they provide the circuitry to make a voltage measurement.

Yup, that’s how I read it. In theory power can flow the other way but you’d need to have supply to turn it on to do that, entering some Schrodinger Power situation.