iOS finally comes with a power saving mode which cannot be a huge help when you are low on battery. The Low Power Mode reduces power consumption and manages some features like background app refresh to save you some energy. To disable / enable power saving mode on iPhone, iPad and iPod Touch devices running iOS 9 or above, do as follows:
1. Open the Settings application on your iPhone
2. Scroll down to Battery settings and open it
3. Toggle on the Low Power Mode to manually enable Power Saving Mode on iPhone.
Note that, the power saving mode will be automatically activated as soon as your battery level reaches to 20%