Comment 25 for bug 32921

Giving some kind of notification a good while before the battery is drained empty (perhaps around 33 %) would be a good thing, since a battery can only handle a certain number of full discharges.

http://batteryuniversity.com/parttwo-34.htm

Of course, users would need to be educated. How about a red notification icon with a tooltip that says “The battery is at 33 % capacity. You still have n hours, n minutes of runtime, but if you have the chance to connect the computer to mains now, it will prolong your battery lifetime.”