Interestingly Apple's old PowerBooks had this. They had a very small internal lithium rechargeable battery. So you could close the lid(making the PowerBook go t

Interestingly Apple's old PowerBooks had this. They had a very small internal lithium rechargeable battery. So you could close the lid(making the PowerBook go to sleep). Then you could swap the main battery without having to shut down the laptop. It could power itself off the internal battery (in sleep mode) for a few minutes.