Fix your iPhone battery’s stuck percentage
Apple has recognized the issue and is supposedly researching a fix that they will include in their next iOS update. On their own support page Apple states, “If you change the time on your iPhone 6s or iPhone 6s Plus manually or change time zones when traveling, you might notice that the battery percentage doesn’t update.” So, that’s what they are saying could be the cause, but don’t have a software fix. So while we wait for that to get pushed out, we can do a little troubleshooting on our end.
Make sure that your iPhone’s data and Wi-Fi are on and connected
Open Settings -> General -> Date & Time
Toggle the switch for Set Automatically Off and On again. The switch will turn gray, then green.