Just got update the iOS to iOS 9.0 for both iPhone and iPad, found that there got one sweet feature added, the Low Power Mode.
Once you’re on iOS 9 and when your battery reach 20% capacity, there will be a prompt-out message for you to enable Low Power Mode.
And when you are on Low Power Mode, you will notice that your battery icon will change its color to light orange-yellowish color instead of red color, indicating you are now in Low Power Mode.
In Low Power Mode, the device will disable mail fetch, background apps refresh, automatic downloads, and lower down graphic motion & details, in order to preserve more energy.
You may also manually enable Low Power Mode from Settings > Battery.
Slide to turn on Low Power Mode. And you will also able to review which Apps suck up your battery juice.
The battery icon will remain this light orange-yellowish color until your phone is charge above 80% capacity, then it will turn back to green color.