Several PowerBook models from the early- to mid-1990s all shared a common removable battery (i.e., PowerBook 140–180). This PowerBook Battery Case was designed to safely hold the PowerBook battery at the time, including its attached “battery door” that was removable, but generally remained connected to the battery to make changing batteries quick and easy.
Until I acquired this PowerBook Battery Case, I had never seen one, despite the fact that I owned a PowerBook 160 in the early 1990s. The color is greenish gray and contrasts slightly from the laptops available at the time, but matches the tint of an Apple battery charger for the same batteries I also have in my collection.
My guess is that this battery case shipped with the PowerBook 180c (which I acquired along with this battery case). The Macintosh User’s Guide for the PowerBook 180 states:
“Important care and safety instructions… Transport batteries either inside the computer or in the protective case provided with each battery. Do not transport unprotected batteries.”