The living energy will eventually fade away, even though an electret would provide power eternally. There might not be a balance achievable through a living energy battery, and a natural electret (natural, however not alive).
The energy from living batteries can be amplified to astounding proportions in the ship technology, it will be okay if the batteries have to be changed once-in-a-while.
My reasoning as to why living batteries vs. eternal electrets are to be used in a ship is because of the need to protect the Human energy field.