My T450s Thinkpad has two built-in batteries--one hot-swappable and one inside the laptop. Lenovo calls the former "external" and the latter "internal". When not plugged in, the external battery provides charge until it hits a 5% low water mark, at which point the laptop starts using the internal battery.
What I want: (1) to control from Debian Linux which battery the laptop draws power from at any given time while not plugged in, and (2) to set a value of my choice for the low water mark for the automatic switchover to the other battery, as I think 5% is too hard on LiIon cells. The current firmware from Lenovo is no help--I don't see any setting even remotely related to these problems.