Ken,

The four batteries already mentioned (Johnston, Southside, Neblett and United) are shown as comprising Smith's Battalion at Drewry's Bluff in the OR for July, October and December 1863 and February, March, May and September 1864.

A report of the heavy artillery in the OR dated 2 January 1865 (Vol.42, Part 3, p.1354) shows the following - Johnston Battery serving at Battery Dantzler, Southside Battery serving at Battery Garland and the United Battery serving at Battery Wood, all under Smith's command. His remaining battery, Neblett, is shown as still being at Drewry's Bluff although there is a note stating that it still belonged to Smith's Battalion.

I think it's safe to assume that the four batteries comprised Smith's Battalion until the 1st Maryland Battery was added, presumably on the retreat to Appomattox, although the January report does show the Maryland Battery at Drewry's Bluff.