Very bad in cold weather

[Crzyrio]

the odd part is that it seems to happen everytime when the battery is around 40%. when you try to turn iphone on, it says the battery is empty, but back at home, the battery has 40% left.

Apple likely uses more advanced battery management than this but usually the current % of life remaining in a battery is indicated by the voltage of it at any moment in time.
So lets take an example:

We have a battery ranging in voltage from 0-10V. And for simplicity lets say at 4V the battery is said to have 0% battery and 10V has 100% battery for the iPhone.

The device(iPhone in this car), requires a minimum of 4 Volts to operate hence it is the 0%.

Now at 25 Celsius(77 Fahrenheit) the battery, chemically, is operating smoothly and at 40% battery has a voltage of 6.4 and can continue to operate down to 4.

Now when you move it to -10 Celsius(14 Fahrenheit) the battery chemicals are getting cold and the electrons start to move slower. This now results in the voltage dropping proportionally. So at 40% your battery could now be reading 4 Volts, which is not enough for the iPhone to continue operation.

But above 40% it still has more than 4 Volts so it can continue to operate. That is why it always happens at 40% if you were to put it into a significantly colder environment it might always cut off at 50% since the drop would be larger due to the cold

It is not an easy task to estimate battery current state and a lot of assumptions are made in the above analogy to better explain what is happening but I hope it makes sense.

 

[ZebraDude]

more or less this was my question, I have seen the hot phone warning message, but never caught a cold one.

Thanks

FYI: The Iphone will dim the screen on the iPhone and shut down the charging circuit when it gets too hot before it shuts down.

 

[Newtons Apple]

Apple shuts it down when too hot to protect the iPhone. Too cold will not harm it but the battery will decrease in capacity to the point it will shut itself off.

I have been on snow machines all day long at 17 below in Wisconsin and if you keep it close to your body it will still work. Biggest danger is letting the phone get cold and then bring it inside and if the humidity is high it will sweat like crazy, sometimes enough to set off the moisture indicators.