That's not always practically possible, some games don't make this a tunable, and of course if
they are not open sourced, good luck on having this changed.
And how about users ? Do they have to remember they're running on battery and fix settings
accordingly ? Or do we have to make all applications of this kind 'PM aware' ?
And by the way, fixing the FPS has drawbacks.
Let's imagine you have a variable frame rate of 20 to 50 at lowest frequency depending on your
position/viewpoint in the game. Capping the frame rate at 50 and running at higher speed does
not guarantee that you'll play longer. Capping at 20 does not give you the same visual result
at all.