Question: I would like Low Power Mode to be the default state of my device.

When I get to 80% battery it toggles itself off automatically.

How can I stop it turning itself off?

How can I always keep my phone in low power mode (or some other equivalent)?

Answer: 
With ios13, you can make an automation that turns on low power mode when your phone turns it off. 

![View screenshot](https://i.stack.imgur.com/XOpasl.png)
You can configure most of your phone's settings to be the same as their Low Power Mode equivalents. This includes turning off email fetch, background app refresh, automatic downloads, and iCloud photo sync. As well as setting screen lock to 30 seconds and keeping the brightness low. 

[List of Low Power Mode settings](https://support.apple.com/en-us/HT205234)