Yeah, mine never did that before. It's crazy. I will just use that other youtube app for now which I can watch videos all day and it's fine.
I've been having the same problem with my iPhone SE. My dad told me to book an appointment with the Genius Bar (which I already had a bad feeling about). When I told the employee about the battery draining extremely quickly, e.g. 10% less after watching one video, and that the phone was getting very hot, he told me it's due to my usage of the phone. He told me I must be viewing them at 1080p which I never do, I always watch videos on YouTube at 720p or less. He didn't even suggest that it might be due to a software/app/update issue, he was intent on blaming my usage on the battery life problems. I've never experienced this problem before and my viewing habits have not changed. After researching this problem online I've found many threads, posts and videos complaining about poor battery life and YouTube app causing sharp battery draining since the iOS 11 updates.
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Official YouTube app is awful. It’s been a a battery hog for a few years so it’s not iOS11.
It’s known to play silent background audio and kill your battery.
Lots of people have not been experiencing this issue with the YouTube app in the past, for many people it has been since the new iOS 11 updates that battery draining problems have become an issue.
I still use my iPod Touch which is on iOS 9.3.2, yet the YouTube app never drains my battery like it does on my iPhone SE. My iPod battery can last me two days despite using YouTube quite frequently. In addition my YouTube app on my iPod is on the latest update, the same version as the YouTube app on my iPhone so it seems likely that it may be an incompatibility with iOS 11 or a software/bug problem with iOS 11 itself.