I know this been a thing for some while but for the first time I've noticed an iOS update has had a detrimental effect on my year old iPhone SE. In fact it's pretty much destroyed it. Prior to the update it rarely went below 85% after an overnight charge but since the update I can run the battery flat in a less than an hour using something like Google maps. I was going to make a warranty claim until Google returned nearly 7 million hits for iOS 11.4 killed my battery. If you haven't already upgraded, I'd wait.