It doesn't need to replace conventional charging. But a phone that gained charge when unused might still be useful - being able to make a call later might be better than never.
It would be great if used as an "emergency power source". I see this equivalent to the satellite SOS systems - not useful for daily use but essential in emergencies. You know you can get brief emergency function every x hours.
But for the question asked higher in the thread, it would never be comparable to a "real" battery. This would be a very special purpose battery.
But for the question asked higher in the thread, it would never be comparable to a "real" battery. This would be a very special purpose battery.