Dual batteries: my X250 on Linux, I'd like to set it up so when the external battery gets to 20% or less, the internal battery starts draining and the external no longer discharges. Can't work out how to do it. Tried Gnome/KDE and Debian/Fedora. Anyone cracked this?

PS: Mine was £100 as well, seems to be the going rate here in UK or was.