Symptoms
- Phone powers off unexpectedly at 20%, 30%, or even higher
- Shutdowns happen more in cold weather or during demanding tasks like the camera
- Phone turns back on immediately when plugged in after shutting down
- Battery Health shows reduced maximum capacity
Likely causes
- CommonAged battery unable to supply peak currentWhen the battery can't meet a sudden power demand, like opening the camera, the phone shuts down to protect itself.
- CommonCold temperature reducing battery outputBatteries deliver less current in the cold, which can trigger shutdowns even at moderate percentages.
- PossiblePeak Performance Capability already reduced by iOSiOS may have already throttled performance to prevent shutdowns; continued shutdowns mean the battery has degraded further.
- Less commonCorroded or damaged battery connectorPhysical damage to the connection can cause intermittent power loss.
What to do
- Check Settings > Battery > Battery Health & Charging for maximum capacity and any performance management message.
- Note the temperature and activity when shutdowns happen, since cold and demanding apps are common triggers.
- Keep the phone above freezing when possible, especially before using the camera or GPS outdoors.
- Update iOS to the latest version to ensure current battery management is in place.
- Force restart the phone if it doesn't power back on right away.
- Avoid letting the battery run down to very low percentages, since that's when shutdowns are most likely.
- If shutdowns happen at 30% or higher, treat this as a strong signal of battery wear rather than a fluke.
- Back up the phone regularly in case of an unexpected shutdown during use.
When to call a pro
Frequent unexpected shutdowns at non-trivial battery levels are a standard reason for a battery replacement at Apple or an authorized repair shop.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.