From Tim Hardwick at MacRumors:
Most iPhone users are familiar with the way iOS throws up a prompt to turn on Low Power Mode when the battery reaches 20%. Low Power Mode can also be turned on manually at any time by going to Settings -> Battery and toggling on the switch next to Low Power Mode.
If you find yourself constantly having to turn on Low Power Mode late in the day, you can save some time by setting it up to run automatically when your battery is depleted to a specific custom level. This can be done in the Shortcuts app, as the following steps show.
Been doing this on my iPad for years – 30% is my trigger point.
