Open the roll-top pocket
Unroll the pocket and open the velcro at the top. Slide in the battery. It maybe a little snug, but that's ok.

Horizontal fixation of the battery
After sliding the battery all the way down, close the horizontal webbing with the velcro.
Closure of the roll-top pocket
Close and fasten the top velcro.
Roll-up the roll-top pocket
Roll up the tube until it sits to secure the battery.
Fix the battery
Buckle up the left and right side and pull the webbing for a tighter fit. The height of the pocket is now in set-it-and-forget-it-mode: You can release the battery simply by opening up the buckles on the side of the pocket, no need to re-roll the top part!