Hexamer returned to command in the fall of 1863. His battery served in the Reserve Artillery in the Bristoe Campaign and the Battle of Mine Run. In 1864, Hexamer’s battery continued in the Reserve Artillery in the Overland Campaign until the Battle of Cold Harbor, when it was assigned to VI Corps. The battery supported the failed attacks of the Union army at Cold Harbor. The Battery remained with VI Corps in the early stages of the Siege of Petersburg.