One way to conserve power on your iPhone when the battery level gets uncomfortably low is to turn on the built-in Low Power Mode. This feature reduces power consumption on your iPhone by doing things like reducing your screen brightness, slashing background tasks, and making your iPhone fall asleep faster, among other things.
On the other hand, iOS doesn’t kick itself into Low Power Mode automatically until you reach at staggeringly-low 20% of your battery life. By that point, you’re already grasping at straws. That’s why iFlames released a new free jailbreak tweak called Low Power Modder in Cydia.