Reserve battery on iOS devices?

A few years ago, one of my friends told me that you can access the extra 20 percent of battery from your iPhone, and he did it though MobileTerminal, does anyone know what to type (after getting into super user) and what to install in Cydia?

If they have it they have it for a reason which i would think would be for date and time and maybe other things so i think its bet not to touch it

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