Why did my battery icon go red or yellow?
Why did my battery icon go red or yellow?
iPhone 12, iOS 15
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See the following from Use Low Power Mode to save battery life on your iPhone or iPad - Apple Support
When Low Power Mode is on, the battery in the status bar will be yellow. You'll see a yellow battery icon 
and the battery percentage. After you charge your iPhone or iPad to 80% or higher, Low Power Mode automatically turns off.
Red indicates the battery is very low and needs to be charged.