The thing is that when an external slice battery pack is connected, the laptop first discharges the internal one. This is a problem because then it is impossible to switch between multiple external battery packs without powering off the laptop. It is also not possible to just disconnect the external battery pack when you e.g. need to insert it into a docking station.
Thus, my question: how to change the discharge order under Linux?
For my use cases following order would make much more sense: when an external slice battery is connected, first discharge it completely before using power from the internal battery.
Background: For some Thinkpad models (e.g. X220) there are slice battery packs (example) available that you can connect to the bottom of the laptop (similar to a docking station).
They seem to be supported well on Linux - e.g. the Gnome Shell battery applet includes both batteries (internal and external) when connected - including displaying the current capacity for each.