Received an error message trying to enable camera (photo/video) flash when the iPhone was below 20%, even after I disabled Low Power Mode. Only occurred in one instance; subsequently issue did not arise when battery was low. Why does it occur only sometimes and is there a way to prevent flash being disabled? In some circumstances I'd prefer to sacrifice battery for a photo/video with flash. Really don't understand why such a limit is forced.
iPhone 7, iOS 11
It is probably designed to prevent your device from powering off.
If a battery cannot provide enough current to the processor, screen and the other active hardware it can cause the device to shutdown unexpectedly, potentially causing data loss (if it was in the middle of doing something). If Wi-Fi, Cellular, Bluetooth, camera are all active that can be a substantial load on a battery, add the flash LED's & other circuitry it may be more than the battery can provide at that moment.
This may occur if you are in cold conditions as batteries can perform poorly under low temperatures.
Next time it happens grab a screenshot of the dialog & consider taking it along to an Apple store if you need help or a better explanation. They may be able to test the battery to see if it is in good condition.
You also have to consider that certain live effects or photograph styles may require more processing - so if you take a 'portrait mode' photo (which uses 3d depth mapping and filters) it may add to the CPU/ Graphics processor load, requiring more power.
This was released today…
https://support.apple.com/en-gb/HT208387
See the "Preventing unexpected shutdowns" section
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During the most extreme cases, the camera flash will be disabled as visible in the camera UI
Flash disabled on low battery
