If it can display a bitmap, why not accompany it by text? "Battery critically low. Connect to charger." Is it that hard to do? Apple is progressively dumbifying everything to look minimalistic. When you don't know why your device is not turning on, being cryptic and minimalistic is not helpful, more verbosity helps.
I remember a thread here where an iWatch user was presented with a red sign with an exclamation point. No t3xt. WTF does it mean? Why we should decrypt Apple's gobbledygook?!