The invention provides an assembly comprising a holder (16) and a portable device (15) having a fluid-fillable container (4). The holder (16) is adapted for holding at least the container (4) and comprises a system (20) for filling the container (4) with a fluid from a fluid source (19). The holder (16) further comprises a detector (3) for detecting the fluid level inside the container (4). The invention further provides an assembly comprising a fluid-fillable container(4), a light source for at least locally illuminating the interior of the container (4) and an optical detector. The detector is arranged outside the container (4) and is adapted for detecting a fluid level inside the container (4) by detecting light scattered inside the container (4) resulting from the light emitted by the light source.