The marker for Battery K, 5th U.S. Artillery is on the north side of Boonsboro Pike east of Rodman Avenue. (Boonsboro Pike tour map) The battery was armed with four 12-pounder Napoleons and was assigned to the 2nd Division of the 5th Corps. 

From the marker:

U.S.A.

Battery K, 5th U.S. Artillery 

Lieut. William E. Van Reed, U.S.A., Commanding.

(September 17, 1862.)

Battery K, 5th U.S. Artillery, crossed the Antietam by the Middle Bridge and about 2 P.M., September 17, relieved Battery M, 2d U.S. Artillery, taking position on the ridge immediately north of this point. After the expenditure of 400 rounds of ammunition at 5 P.M., the Battery was relieved in turn, by Battery M. 2d U.S. Artillery, soon after which it recrossed the Antietam.

No. 97.

See more on the history of the 5th United States Artillery, Battery K in the Civil War.