Switch to Low Power Mode when your iPhone battery is low or when you may not have access to electrical power. Low Power Mode limits background activity and tunes performance for essential tasks. Using Low Power Mode can significantly increase the life of the battery charge.
Turn on Low Power Mode. Go to Settings > Battery, then turn on Low Power Mode.
Note: If your iPhone switches to Low Power Mode automatically, it switches back to normal power mode after charging to 80%. Your iPhone might perform some tasks more slowly when in Low Power Mode.