On Android, users can pre-set the amount of battery life that remains when the Battery Saver mode is activated automatically. Some will pick 15% or lower. However, some sporting a handset running Android 9 found that level set at 99% (not by them), which essentially turned the Battery Saver mode on instantly on these devices.
As Google explained on the Google Pixel Reddit page, this issue was accidentally caused by "an internal experiment to test battery saving features that was mistakenly rolled out to more users than intended." Google has since returned the Battery Saver mode to default settings. And while Google did say it was sorry, some of those affected were upset that Google had changed settings on their handsets without notifying them. But what these users aren't grasping is that they weren't supposed to be involved in the experiment in the first place, so there was no reason for Google to alert them about it in advance.
"Hi all, some of you may have noticed that battery saver turned on automatically today. This was an internal experiment to test battery saving features that was mistakenly rolled out to more users than intended. We have now rolled battery saver settings back to default. Please configure to your liking. Sorry for the confusion."-Official Google Account
So with everything returned to normal, you shouldn't worry anymore about the Battery Saver mode activating randomly on your Pie powered Android phone.
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