iPhone 5S suddenly turns off at 30-40% of battery life

Solution 1:

When I was a Genius at an Apple Store about two years ago, this issue was relatively common. Most likely this is a hardware issue with the battery itself, but there is a chance it could be a battery calibration issue. It's best to troubleshoot calibration first.

Calibration Issue

The battery itself keeps track of how much battery life it has, and reports this back to the phone, which it then displays on the screen for you. Sometimes, the battery may be fully charged, but the battery or phone instead reports a different charge percentage.

Fixing a calibration issue is relatively easy:

1. Use your phone until it completely runs out of battery power and shuts off.
2. Plug your phone into a charger, and leave it plugged in until it is fully charged. Ideally, do this overnight so you know to be sure it gets completely charged.

This process will tell the phone and battery where its lowest and highest charge points are.

Hardware Issue

If you continue having the issue, it is almost certainly an issue with the battery itself. If you have an Apple Store nearby, make a Genius Bar appointment there and bring your phone in. They can replace your battery there the same day, and it usually takes less than an hour.

If you don't have an Apple Store nearby, there may be an Apple Authorized Service Provider near you that can help. Go through the prompts at Apple's support site, and they will help you find a good option.

Solution 2:

Battery percentage is usually an estimation as it can be quite difficult to work out actual percentage.

A great way to see the health of your battery is by using a tool such as CoconutBattery (Mac, free) which will allow you to view the cycle count, current capacity and how much it can store of its original design capacity.

Remember, batteries are considered a consumable and once they've been cycled too many times will need to be replaced.

This could also be a software issue as some other users have mentioned, so your best bet is to backup the device then restore it through iTunes, setting it up as new to test.

Solution 3:

I'm experiencing the same problem and believe iOS 10 software is the cause and not hardware.

If iOS 10 is not registering the correct amount of battery power. Occasionally prior to my iPhone 6S going to sleep it's registering a decent amount of power to wake from sleep (as much as 50%). The phone will not wake from sleep and is effectively useless. However connect it to a power source or a computer, the power issue resets and the phone will wake reporting over 30% power and more. It seems to reset some flag that tells the phone that the battery is dead. It has happened too many times to be a coincidence. iOS is not registering the correct battery level, mistakenly thinks it does not have enough power and will therefore not power up. Once connected to a power source for a few seconds, the phone will reboot normally and show power levels correctly.. This has occurred through all iterations of iOS 10 including the latest betas.