The battery can be found in the same room as the blue beam that's used to unlock the
achievement near the start of the game. It can be found in between the blue beam and the creepy doll sitting on the red box, closer to the doll. To get to this room, from the beginning, move left past the foul smelling door and the shelving unit and interact with the second door you see using
. The battery will light up when you're near it. Hit
to pick it up and the achievement will unlock.