The batteries housing is held together by the two teeth on the assembly front; the one tooth at the back, that keeps the two halves together; the one screw that holds the main circuit board to the top half of the batteries housing; two pin connectors; and, sadly, the nickel strips that are welded to the battery terminals;

In this step:

unscrew the circuit board screw (arrow in the 1st photo)

bend the two teeth at the front of the assembly (indicated by arrows in the 2nd photo)

unplug the pin connectors 1 and two (circled in the 1st and 2nd photos)

undo the two halves of the battery assembly on the side opposite the circuit board

and, as carefully as possible, break off the strips welded to the battery terminals. NB: do not short the battery + and - terminals with your tool while working on it

for access to the top terminals, unscrew the translucent plastic cap that covers it.