Assigned to the Army of the Potomac's (AoP's) Artillery Reserve During the Chancellorsville Campaign,the battery fought in the Second Battle of Fredericksburg, May 1–4, 1863, without sustaining any losses.

As they waited in reserve, the men of the battery noted that all was quiet along the lines, with no firing to be heard except musketry of distant pickets and, at long intervals, occasional cannon shots.

That changed at 4:00 p. m. when Captain Hart received orders to take the battery to the front and into position on Maj. Gen. Sickles' III Corps line.

General Hunt, AoP's Chief of Artillery, placed the battery at 4:30 p. m. in the Peach Orchard along the Wheatfield Road, with the four guns pointed south.

[6] From here, they opened on the enemy's artillery, using solid shot and shell with such good effect that one of the opposing batteries was forced to withdraw.

During the lull after the second attack, Hart reported to McGilvery that they had exhausted their stock of canister, and had nothing left in the caissons except some solid shot.

It then limbered to the rear of the main line where it repaired damages, gathered new horses, and replenished the ammunition chests, after which it reported to General Tyler that it was again ready for action.

When the Confederate infantry advanced, at the close of the artillery fire, the battery assisted in the repulse of Wilcox's and Perry's brigades, using double charges of canister on their ranks.

[9] During the artillery fire with which the enemy sought to cover the retreat of their infantry, the 15th New York Battery achieved signal success by exploding two of his caissons and dismounting two guns by well-directed shots aimed by Sergeant Sheehy and Corporal Hammond.

Lieutenant Knox, who was severely wounded, is mentioned in Captain Hart's report as one whose " noble and gallant conduct deserves the highest honor that could be conferred on him.

After Gettysburg the battery participated in the fall Bristoe and Mine Run campaigns, notably being present at Rappahannock Station on November 7, 1863, after which it went into winter quarters with the AoP near Culpeper, VA.[10] Breaking camp early on the morning of May 5, 1864, Captain Hart and his command moved with Grant's army on the Wilderness campaign, during which it was present or engaged in that long series of hard-fought battles.

During the assault on Petersburg, June 17, 1864, the battery was actively engaged, its guns being run up well to the front and placed within easy canister range of the enemy's earthworks and artillery.

Captain Hart and his men received special mention in the official reports for the courage and skill with which they handled their guns on this occasion.

In addition to the losses sustained in this action, several men were killed or wounded in the trenches before Petersburg during the long siege that followed this first assault.

On the next day, August 20, the artillerymen busied themselves in throwing up earthworks in front of the guns, putting down platforms, and strengthening their position generally.

It forced the Confederates to carry their supplies 30 miles (48 km) by wagon to bypass the new Union lines that were extended farther to the south and west.In the battle at the Weldon Railroad the Hart's Battery lost 11 men killed or wounded; a severe loss, as it only had 4 guns at this time.