Is there any way to calculate and show the rate of the battery drain?
Using xterm you can see the percentage of battery you have left, and if it also shows the exact amount of charge left in the battery then it would be possible to calculate battery drain..
That way you'd be able to see if your battery is draining too fast, or when you're low on power you can check and see how much you're spending and know how much time you have left approximately.