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: 
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)
There is no way to achieve this functionality as it will always turn off at 80% as the purpose of it is to extend your phone when at low battery.

From [this Apple Support discussion:](https://discussions.apple.com/thread/7268548)

> 
> Low Power Mode reduces or turns off Email fetch, Hey Siri, Background
>  app refresh, Automatic downloads, Wi-Fi associations, and some visual
>  effects. If you can live without having those features available on
>  the phone, you can keep it on low power mode. But after you charge
>  your iPhone to 80% or higher, Low Power Mode automatically turns off,
>  so you'll have to turn it back on again.
> 
> 
>