Okay. What about the possibility for the user to calculate graphically the left time with the actual Battery usage?

It should be easy to use. In my mind it's only a line with a fixed point at the end of the actual graph (percentage). The user can move the left side (blue pinhead) of the line and can change the angle of the line resulting a intersection on the other side of the line with the x-axis. Battery Graph shows the exact time the Battery reaches 0% with the angle the user has given.

I have drawn a fast mockup. I hope the developer like the Idea: