I have been given a 6s iphone about which I know very little. I have updated the software and fitted a new battery followed by full charging and discharging as recommended. I fiddle albeit carefully with all or most of the settings, using a 388-page guide book. Sometimes but not always when I am using the camera for simple single shots, no flash, the 'switch off, slide button to the right to switch off' with a white Delete button below, comes up every 20 seconds or so, as though to perhaps protect the battery. But the battery can be showing 80% or so and I think it is ok. The old battery warning showed 74%. The phone doesn't switch off, It ask me if I want to. Auto Lock is set at 5 minutes. I suspect that the same thing happens at other times but I can't give details. I would be grateful for any advice.