From T.T. O’Brien– Low Power Mode:
This is remarkable. You get a warning to go to Low Power Mode when the iPhone’s battery charge hits 20-percent, but I charged my iPhone overnight to 100-percent, unplugged it the next morning, and turned on Low Power Mode. That cuts down on background activities, turns Mail from Push to Fetch, puts Siri into manual mode, but text messages still arrive on time.
It works. Try it.