SUMMARY There's an interesting article on prolonging battery life at: https://batteryuniversity.com/index.php/learn/article/how_to_prolong_lithium_based_batteries They recommend: "To make this feature user-friendly, a device should feature a “Long Life” mode that keeps the battery at 4.05V/cell and offers a SoC of about 80 percent. One hour before traveling, the user requests the “Full Capacity” mode to bring the charge to 4.20V/cell." So, it is basically my wish that this is implemented in KDE. You can't do that now, right? If not, it seems like Powerdevil would be a nice place to have a setting that limits the charge to a certain percentage of full, and then possibly a plasmoid or something that makes it really easy to override that like they suggest. If I understand it correctly, there is stuff in TLP that can help with this, at least on some system, but I don't know if that's the appropriate way to implement it.
As far as I know, only Lenovo laptops support this feature. However there is an open patch to implement it, happily: https://invent.kde.org/plasma/powerdevil/-/merge_requests/5
MR https://invent.kde.org/plasma/powerdevil/-/merge_requests/5 was merged in Plasma 5.20