The marker for Batteries B and L, 2nd U.S. Artillery is on the north side of the Boonsboro Pike, east of Rodman Avenue.  (Boonsboro Pike tour map)

From the marker:

U.S.A.

Batteries B and L, 2nd U.S. Artillery 

Captain Jas. M. Robertson, U.S.A., Commanding.

(September 17, 1862.)

Horse Batteries B and L (consolidated), 2d U.S. Artillery, crossed the Antietam by the Middle Bridge in the forenoon of September 17, and went into position on a knoll 80 yards south of this point and engaged the enemy. It was soon disabled by the fire of the Confederate Artillery from Cemetery Hill and withdrawn, its position being taken by Battery E, 1st U.S. Artillery. One section of the Battery, under command of Lieut. Albert 0. Vincent, moved to the northwest about 860 yards and engaged a Confederate Battery posted in the northern part of Piper’s Orchard. The Battery was withdrawn before dark and returned to its bivouac near Keedysville.

No. 94.

See more on the history of the 2nd United States Artillery, Batteries B&L in the Civil War