Q: iphone 6 s keeps switching off with battery life remaining

Yes at some point below 20% battery charge life, the device will begin to shut down. It is behaving as designed to prevent damage to it. In a pinch, you can switch to Low Power Mode (Settings app > Battery Low Power Mode), but that will just buy you a little time. The solution it to recharge it.

Try recalibrating the battery:

Charge to 100%.

Run the battery down until the phone shuts off.

If it shuts off above 1% charge it for a minute only, then continue running the battery down. Repeat until it makes it to 1% before shutting off.

Charge it to 100% again.

From :     6s battery replacement? Apple says my phone is fine.

You could also download the app Battery Life from the App Store and test your battery with it. It is reliable.

Kilgore-Trout wrote:

 

Yes at some point below 20% battery charge life, the device will begin to shut down. It is behaving as designed to prevent damage to it. In a pinch, you can switch to Low Power Mode (Settings app > Battery Low Power Mode), but that will just buy you a little time. The solution it to recharge it.

Actually, it shouldn't shut down unless the battery is failing or the battery gauge needs to be recalibrated. It should continue to work down to about 1%, with warnings at 20% and 10%. And even when it shuts off below 1% there is still energy in the battery; this is a safety feature designed to protect the battery from going completely flat.

Admittedly, I very rarely let my devices get below 10%, but I have indeed seen the auto shut off cycle (not the warnings) start at well above 1%

None of my 5 iPhones and 2 iPads have ever switched off above 1%, except for one 4S, and that one had a bad battery that Apple replaced. If yours does try recalibrating the battery gauge, and get Battery Life to check the condition of the battery. The battery gauge is an estimate, and is only calibrated at full charge and near flat. That's because Lithium chemistry battery voltages do not vary measurably over the discharge cycle, except at the top and bottom of the range. So the charging circuit knows when the battery is full because there's an uptick in voltage, and knows when it is low because there's a sudden dropoff in voltage. If that dropoff occurs anywhere above 1% the phone will shut down regardless of the battery gauge display. In between those two reference points the battery gauge monitors the discharge rate and estimates the remaining charge, but if you never run it all the way down, over time it will get out of calibration - especially if you never fully charge it. Apple used to recommend going through a cycle of charging to 100%, running it down to zero, then charging to 100% once a month. They dropped that recommendation, probably because it was more often than necessary, but when it was there it was to recalibrate the battery gauge, not to cycle the battery.

I have had iOS devices for years, Lawrence. I am aware of all that. And I have no need to re-calibrate battery gauge. As I said, it is extremely rare I let the devices get below 20%. I suspect, however, that the shut off mechanism is based not only on the current charge level, but the rate of discharge (use) at the time as well.