the most favourable from a component layout and ease of manufacture point of view was to put a large, flat battery under the keyboard
the removal of the battery from under the display has opened up space to use a supercap to assist the battery
We hatched an audacious plan to squash the keys when the device was shut, saving 1.4mm in thickness. This is done with magnets on the corners which compress the keys and hold the lid and keyboard frame together.