Animated diagram highlighting the main (red) and secondary (blue) batteries of the battleship USS Washington.
Generally used only in the terms of naval warfare, the main battery is the primary weapon around which a ship was designed. "Battery" is in itself a common term in the military science of artillery. For example, the United States Navy battleship USS Washington had a main battery of nine 16-inch (410 mm) guns arranged in three turrets, two forward and one aft. The secondary battery was 5-inch dual purpose guns, meaning they could be fired against other ships or used as an anti-aircraft weapon. The term also refers to smaller ships, like heavy or light cruisers.
Cut-away illustration of the 16 inch gun turret on Iowa class battleships that formed the primary battery.
Often, ships had a primary battery for offensive purposes, and a secondary and sometimes even a tertiary battery for self-defense. An example of this was the German battleship Bismarck, which carried a primary battery of eight 15 inch (380mm) guns, along with a secondary battery of twelve 5.9 inch (150mm) guns for defense against destroyers and torpedo boats, as well as a tertiary battery of various anti-aircraft guns ranging in caliber from 4.1 inch (105mm) to 20mm guns. Many later ships during World War II used dual-purpose guns to combine the secondary battery and the heavier guns of the tertiary batteries, in order to simplify the design.
Newer vessels are designed with anti-ship missiles, replacing heavy naval artillery designs. An excellent example would be the Soviet/Russian Kirov class rocket cruisers. In the 1980s the four battleships of the United States Iowa class were fitted with Tomahawk and Harpoon missiles, their 16-inch guns were still retained.
The original article can be found at Main battery and the edit history here.