On the D100 Series EMS devices -- including the FlightDEK-D180 -- each graphical widget is dedicated to a single purpose. You can choose not to display that item, but its space on the screen cannot be devoted to another purpose.
With the D180, you do have the option of adding an Info item to the flight instrument display. Each of two areas -- in the upper left and right portions of the display -- can be populated with one of 5 Info items. Among those 5 is the option to display External and Internal system voltage.
Refer to page 5-11 in the D180's Pilot User Guide for details on those Info items.