The shield battery is a protoss support structure. Its core contains a khaydarin crystal that absorbs and stores local psionic energy. The stored psi is transferred to protoss units via the energy spokes, allowing plasma shields to be recharged rapidly. While the battery has a limited energy capacity, it will steadily recharge itself over time.
The shield battery appears in the campaign of Legacy of the Void as a basic building requiring a Nexus. Their shield recharge can be set to autocast, which makes them considerably more useful. However, they only restore the shields of defensive turrets when set to autocast, not other buildings.
Blizzard has stated that they are looking into bringing the campaign shield battery into Legacy of the Void multiplayer, as a 75 mineral structure that can recover allied shields even as they are in combat. The addition of the shield battery is currently being tested in the testing matchmaker.
The following section contains information from a previous version of StarCraft II which is no longer valid.
It appears that the shield battery was once part of the StarCraft II game engine. However, Karune has said the shield battery will not be available in multiplayer games. It did not appear in the Wings of Liberty product.