Michael:
The 20th Virginia Heavy Artillery Battalion was formed on June 21st, 1862 based on a company of heavy artillery commanded by Captain James Robertson that had previously served at Norfolk, VA. It had four companies with a fifth company added sometime later. The company commanders were A-Robertson, B-St. Bride, C-R C Jones, D-R K Hargrove, E-W J Dabney. The battalion was commanded by Major James E. Robertson. As the fighting approached Richmond, the battalion became part of the defenses of that city. It served in the Second Division Heavy Artillery which was commanded by Lt. Col. James Howard. In May, 1864, the battalion's position was in the intermediate defensive line of Richmond with 3 24 pdr Howitzers, 2 32 pdr Siege guns, 1 8" howitzer, 4 6 pdr field guns, 5 24 pdr siege guns and 1 12 pdr howitzer and possibly additional cannon. They were in positions along the Mitchell's Tract Road, Brock Road, Mill Road, Deep Run Road and the Plank Road. I believe these roads to be near Richmond if so, this would mean the battalion did not serve at Drewrey's Bluff. You can locate these roads on a map of Richmond. After the fall of the Richmond, the battalion continued service as infantry and surrendered at Appomattox with only eleven men still in the ranks. Above information from various volumes of the "Official Records" and my files. If you continue to search for information, with confederate artillery they were know by the name of their commander as much as their official name. So remember to search Major James E. Robertson as well. Hope this may assist you.
Ron