iphone 8+ repeated sudden shutdown

Hello AlloaLes,

Thanks for choosing the Apple Support Communities. We understand your iPhone is shutting down and not showing the correct battery percentage. It sounds like this may be a battery health issue. Let's check your battery health and see if the battery needs serviced. Here is how.

Check the iPhone battery health and usage - Apple Support

"View your battery usage information

Go to Settings  > Battery.

Information about your battery usage and activity appears for the last 24 hours and up to the last 10 days.

• Insights and suggestions: You might see insights about conditions or usage patterns that cause iPhone to consume energy. You might also see suggestions for lowering energy consumption. If a suggestion appears, you can tap it to go to the corresponding setting.
• Last Charged: Indicates how fully the battery was last charged and the time it was disconnected.
• Battery Level graph (in Last 24 Hours): Shows the battery level, charging intervals, and periods when iPhone was in Low Power Mode or the battery was critically low.
• Battery Usage graph (in Last 10 Days): Shows the percentage of battery used each day.
• Activity graph: Shows activity over time, split by whether the screen was on or off.
• Screen On and Screen Off: Shows total activity for the selected time interval, for when the screen was on and when it was off. The Last 10 Days view shows the average per day.
• Battery Usage by App: Shows the proportion of the battery used by each app in the selected time interval.
• Activity by App: Shows the amount of time each app was used in the selected time interval.

Note: To see battery information for a specific hour or day, tap that time interval in the graph. To deselect it, tap outside the graph.

Battery life and charge cycles vary with use and settings. The iPhone battery should be serviced or recycled by Apple or an Apple Authorized Service Provider. See the Battery Service and Recycling website.

To learn how to maximize your battery’s performance and lifespan, see the Apple Support article iPhone Battery and Performance."

We hope this helps.

Cheers!