Does not work on the iPhone X unless you have "StatusBar NoX" installed.
Set a custom battery percentage style.
Some examples:
iOS Curved Format (100)
Raw Percentage (94.1%)
Raw Percentage (Ninety-Four)
Battery Discharge (230 mA)
You can display 1 or 2 styles, simultaneously.
You can change the update interval.
The default value is whenever your statusbar clock updates (probably once every 60 seconds if you use a clock format without seconds, 09:24 PM, and probably once every second if you use a custom clock format with seconds, 9:24:53 PM).
BeGreen can change it to once every 5, 15 or 30 seconds.
I can't give you any quantitative data, but obviously, the shorter your time interval, the more impact this will have on your battery. Is having it update once every 5 seconds enough to have any noticeable impact on your device? I don't honestly know.