I found this Information:
"What I found out is that T470 is using a new algorithm which checks the wear level of each battery and first discharges the battery which is worn the least. This makes sure that the two batteries wear out evenly and maximizes the lifespan of both batteries. This behavior is different than T460 and earlier systems which always discharge the removable battery first." says, someotherguy on Lenovo Community.
I also fact-checked this myself as I have a Lenovo T550. The info is valid.
So it is likely that they are still implementing this in your T480 and my T550.
* There is no known way to switch batteries at the user's behest. Other than removing the external one and praying the internal one has enough juice to allow the insertion of the next.
* You can set the critical battery option though. Doing this is different depending on distro.
Found the Information: Lenovo Community