At the beginning of the war, Governor
Charles Smith Olden and Hexamer offered his battery to the federal government. At first it was refused, but it was added to the volunteer service after a four-month delay, being mustered into service on August 12, 1861. Thereafter it was known as
Battery A, 1st Battery New Jersey Light Artillery. The battery served at first with
First New Jersey Brigade of Brig. Gen.
Philip Kearny in the
Peninsula Campaign, where it was part of
VI Corps in the
Army of the Potomac. Hexamer’s battery next saw action in the
Antietam Campaign. It served with VI Corps at the
Battle of Crampton’s Gap and the
Battle of Antietam, assigned to the division of Maj. Gen.
Henry W. Slocum. The battery also served at the
Battle of Fredericksburg with the same division under Brig. Gen.
William T. H. Brooks. Hexamer was ill and missed the
Second Battle of Fredericksburg and the
Battle of Salem Church, both fought by VI Corps. The battery was commanded by Lt Augustine N. Parsons. Battery A covered the Union force’s crossing of the
Rappahannock River on May 3, 1863. The guns accompanied the federal advance toward Salem Church and supported an attack on the brigade of Brig. Gen.
Cadmus Wilcox near the church. When VI Corps was forced onto the defensive, Parsons’ guns supported the infantry line until their ammunition ran low. The battery also supported the federal retreat across the river on May 4. Parsons remained in command for the
Battle of Gettysburg, in which Battery A was assigned to the Reserve Artillery. The battery was sent to the front on July 3, 1863 to resist
Pickett's Charge. Its guns supported the
Philadelphia Brigade to their left front at the crisis of the attack. 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. Hexamer was mustered out of the service on August 18, 1864. Parsons succeeded him in command. ==Post war==