One embodiment includes a bottom clamshell and a top clamshell sandwiching a first battery and a second battery, with a fill port extending from a top surface of the top clamshell through to the bottom surface of the top clamshell and to a space between the first and second batteries. The embodiment includes a protrusion coupled to the top clamshell proximate to the fill port and extending into the space, wherein the protrusion at least partially occludes a direct path through the fill port to the space.