Battery A is referenced on a War department marker at Antietam and honored by two monuments at Gettysburg.

1861
January At Washington, D.C.
Expedition to relief of Fort Pickens, Fla., and return to Washington.
June to August Attached to Blenker’s Brigade, Miles’ Division, McDowell’s Army of Northeast Virginia
July 16-21 Advance on Manassas
July 21 Battle of Bull Run
August Duty in the Defenses of Washington, D.C. attached to Heintzelman’s Brigade, Division of the Potomac
October Attached to Blenker’s Brigade, Division of the Potomac, then Artillery Reserve, Army of the Potomac
1862
March Moved to the Virginia Peninsula.
April 5-May 4 Siege of Yorktown
May 4 Near Williamsburg
May Attached to 1st Brigade, Horse Artillery, Artillery Reserve, 5th Army Corps, Army of the Potomac
May 23-24 Mechanicsville
June 25-July 1 Seven days before Richmond
June 26 Mechanicsville
June 27 Gaines’ Mill
July 1 Malvern Hill
July At Harrison’s Landing
July 31-August 1 Cog’s Point
August 16-24 Moved to Fortress Monroe, then to Alexandria
September 6-22 Maryland Campaign. Attached to Artillery, Cavalry Division, Army of the Potomac
September 11-12 Sugar Loaf Mountain
September 15 Boonsborough
September 16-17
Battle of Antietam

Battery A was commanded at Antietam by Captain John Tidball. It was armed with four 3″ Ordnance Rifles and lost 1 man killed and 3 men wounded.

From the War Department marker at Antietam:

Horse Battery A, 2d U.S. Artillery, moved from its bivouac near Keedysville on the morning of the 17th, crossed the Antietam by the Middle Bridge and preceded and supported by the 4th Pennsylvania Cavalry, went into position on the crest of this ridge about 160 yards north of this point and engaged the Confederate Artillery on Cemetery Hill and the ridge north of it. At noon it was relieved to replenish ammunition. About 3 P.M., it returned to its original position and again engaged the enemy. The right section, in charge of Lieut. A.C.M. Pennington, advanced about 650 yards, to the right and front, took position on the flank of Richrdson’s Division and engaged a Confederate Battery west of the Hagerstown Pike and few yards north of where Piper’s Lane intersects it. The Battery withdrew at dusk and recrossed the Antietam.

September 19 Shepherdstown Ford
November 2-3 Upperville and Bloomfield
November 3-4 Snicker’s Gap
November 4 Markham Station
November 10 Amissville
December 12-15
Battle of Fredericksburg
1863
February 5-7 Operations at Rappahannock Bridge and Grove Church
February Attached to Artillery Reserve, 2nd Division, Cavalry Corps, Army of the Potomac
April 29-May 8 Stoneman’s Raid
June Attached to 2nd Brigade, Horse Artillery, 2nd Division, Cavalry Corps, Army of the Potomac
July 1-3
Battle of Gettysburg

Battery A was commanded by Lieutenant John H. Calef. It was armed with six 3-inch Ordnance Rifles at Gettysburg and lost 12 men wounded and 13 horses killed.

From the monument on Chambersburg Road:

June 30 Arrived in the evening from Emmitsburg and took position on the Chambersburg Pike.

July 1 Advanced with the Cavalry. Went into position with right section on right of the road left section on the left and center section with Col. Wm. Gamble’s Brigade on the right of Fairfield Road. The First Union gun of the battle was fired from right section and the positions held under a severe fire until the First Corps arrived about 10 a.m. The Battery was then relieved by Capt. J.A. Hall’s 2nd Maine Battery and after being supplied with ammunition returned to a line in front of Cemetery Ridge and towards night moved to the left about a mile and bivouacked for the night near the Third Corps.

July 2 A.M. marched with the First Brigade of Major General John Buford’s Division to Taneytown en route to Westminster 

From the monument on Reynolds Avenue:

July 1 Advanced with the First Division Cavalry Corps. The Right and Left Sections on the Chambersburg Pike. The Centre Section under Sergt. Chas. Pergel posted here with the First Brigade First Division Cavalry Corps and assisted in repulsing an attack of the Confederate infantry.

This section having been withdrawn joined the Battery in the rear and again advanced with Left Section and Received Battery B First Maine Artillery on Chambersburg Pike in the afternoon but was soon compelled by a front and enfilading fire to retire. Rejoined the Battery in position with the cavalry on the left in front of Cemetery Ridge and remained during the night.

July 4 Emmettsburg, Md.
July 6 Williamsport and Hagerstown
July 8 Boonsborough
July 10 Old Antietam Forge
July 14 Falling Waters
July 21-22 Chester Gap
September 13-17 Advance from the Rappahannock to the Rapidan
September 13 Culpeper Court House
September 14-16 Raccoon Ford
October 9-22 Bristoe Campaign
October 17-18 Groveton
October 19 Gainesville and Buckland Mills
November 7-8 Advance to line of the Rappahannock
November 26-December 2 Mine Run Campaign
November 27 New Hope Church
February 6-7 Demonstration on the Rapidan

1864

February 6-7 Barnett’s Ford
May 4-June 12 Rapidan Campaign
May 5-7
Battle of the Wilderness
May 8-21
Battle of Spotsylvania Court House
June 1-7 Cold Harbor
June 2 Gaines’ Mill, Salem Church and Haw’s Shop
June 7-24 Sheridan’s Trevillian Raid
June 11-12
Trevillian Station
June 21 Black Creek or Tunstall Station and White House or St. Peter’s Church
June 24 St. Mary’s Church
June 29 Siege operations against Petersburg and Richmond
July 12 Lee’s Mills
July 27-29 Demonstration north of the James River
July 27-28 Deep Bottom
July 29 Malvern Hill
July 30 Lee’s Mills
August 13-20 Demonstration north of the James
August 14 Gravel Hill
August 16 Deep Run
August 16-18 Strawberry Plains
August 23 Dinwiddie Road, near Ream’s Station
August 23-25 Ream’s Station
September 29-October 2 Poplar Grove Church
September 30-October 1 Arthur’s Swamp
October 27-28 Boydton Plank Road
November 7 Reconnaissance toward Stony Creek
December 1 Stony Creek Station
December 7-12 Warren’s Expedition to Weldon Railroad
1865
February 5-7 Dabney’s Mills, Hatcher’s Run
March 28-April 9 Appomattox Campaign
March 30-31 Dinwiddie Court House
April 1
Five Forks
April 3 Namozine Church
April 5 Payne’s Cross Roads, Amelia Springs
April 6 Sailor’s Creek
April 8 Appomattox Station
April 9
Appomattox Court House

Surrender of Lee and his army.

April 23-29 Expedition to Danville
May Moved to Washington, D.C. and attached to Dept. of Washington, 22nd Corps
May 23 Grand Review