Short answers, pulled from the story.
An artillery battery is a company-level military unit grouping multiple artillery systems, such as howitzers, mortars, rocket launchers, or missiles, under a single command. It is designed to improve battlefield communication, command and control, and dispersion of gun crews. In modern armies it typically consists of six to nine artillery pieces and 100 to 200 personnel.
The number has varied across history and armies. In the 19th century, four to twelve guns was typical. During the American Civil War, Union Army batteries usually had six field pieces and Confederate batteries often had four. In the modern US Army, a towed howitzer battery has six guns and a self-propelled battery such as an M109 battery has eight.
In the US Army, a battery is typically commanded by a captain, equivalent to an infantry company commander. In British and Commonwealth forces the battery commander is a Major. Historically, the rank has ranged from lieutenant to major depending on the army and period.
The term originally referred to a cluster of cannons grouped together in action, either at a temporary field position in battle or during a siege. During the 18th century it began to be used as a permanent organizational term in peacetime and wartime. By the late 19th century it had largely replaced earlier alternatives such as "company" and "troop" in most armies.
On a warship, a battery is a group of guns of a particular role or caliber. Warships historically carried a main battery of heavy guns for primary offensive firepower, a secondary battery of medium guns for defense against smaller vessels, and a tertiary battery of lighter anti-aircraft weapons. One example from the World War II era carried a main battery of eight 380 mm guns, a secondary battery of twelve 150 mm guns, and a tertiary battery of anti-aircraft weapons ranging from 105 mm down to 20 mm.
The launch of the all-big-gun battleship in 1906 made every existing battleship effectively obsolete. Previous warships typically carried four large-caliber guns; the new design shipped a main battery of ten heavy-caliber guns, giving it a weight of broadside and rate of fire that could overwhelm any similarly sized warship of the older design.