If you see that your iPhone Battery yellow icon then it means that Low Power Mode is on. This feature is handy in saving on power through putting an end to back ground features such as e-mail check, automatic download and some graphic animation. It is on by default, and when your battery reaches 20% or lower, your device will ask if you want to turn on Low Power Mode manually, you can do it in Settings > Battery. And one of the important functions of this kind is the ability to extend battery life when the device battery is almost depleted.
