After removing the screws, turn right side up again and lift off the plastic cover, exposing 12 NiCd cells in a string, and nothing else.

Note that there is room on the right side for three more cells, which would make a 18V battery pack.

Especially cheaper makers usually have versions for 7.2V/9.6V/12/14.4/18V all using the same plastic, just more or less cells and a different motor.