A toggle "prepare off-grid use", nothing more complicated than that.
You could even try some implicit haptic UI ideas like having two power ports, one for operating (that only charges when the battery is dangerously low) and one for complete charging (that also runs the device, but it's clearly designated as the "charging port"). With USB power delivery that would not even waste case surface, it would be an entirely internal change to make one more USB-C power capable.
You could even try some implicit haptic UI ideas like having two power ports, one for operating (that only charges when the battery is dangerously low) and one for complete charging (that also runs the device, but it's clearly designated as the "charging port"). With USB power delivery that would not even waste case surface, it would be an entirely internal change to make one more USB-C power capable.