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This question already has an answer here:

Today I was walking using my iPhone 6S running iOS 9.3 as a pedometer and it shutdown.

When I got home and plugged it in, it said 30% capacity.

That seems strange. I would expect it for 2% or 5% to preserve RAM, but 30%? Maybe there is a setting I don't know about.

My question is: Why does my iPhone power off when the battery is down to 30%?

Edit: this is not a duplicate of the low battery crash as I witnessed the low power shutdown and it was not a crash.

marked as duplicate by jaume, bmike iphone Oct 8 '18 at 12:58

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    Can be a sign of a dying battery. Try reboot or reset; if no better, get the battery checked out. – Tetsujin May 28 '16 at 10:54
  • A phone powering off at 30% is a big sign of a failing battery. Check your battery at the nearest Apple Service Provider. – Brick May 28 '16 at 10:55
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    I've seen that happen with the phone getting too cold or too hot. I had it shut down on a bicycle ride up Vail pass the other day. The phone effectively froze on me. Similarly when I left it, in its black leather case in the sun. – Deesbek May 28 '16 at 15:17
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This is usually for one of three reasons:

  1. Aging battery. Lithium ion cells chemically age and peak power output decreases as does total charge available to supply at various voltage levels.
  2. Abnormal or failed / out of tolerance battery. Same as above, but the voltage and current is below spec for the battery age.
  3. Bad calibration or hardware. I've never seen that be 30% off, though. After the battery is drained completely, charge the phone back to 100% completely (and for good measure, leave it on the charger for another hour or two). An iPhone can draw more power than a perfect battery can provide due to defect or damage. It might not be the battery.
  • I've had the same issue. Generally 10% give or take, but today it just happened at 30%. I've seen this advice before and it doesn't seem to help me. Maybe it's because my "100%" charges actually get stuck at 97%? – s g Nov 2 '16 at 16:17

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