iPhone 8 battery inconsistency after restart

I have an iPhone 8 and sometimes when I have low battery I will re-start my phone and I then it will go back up to 80% or something. I’m not saying this is a bad thing but sometimes before school I will power it off and it’ll be dead when I turn it back on in the afternoon. I know you can’t really do anything about one persons phone that they own and whatnot but I just thought with the newer phones you should make sure that there’s a test or something so it’s more Convenient for other users.

[Re-Titled by Moderator]