Shield battery

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.

StarCraft II
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.

Co-op Missions
Karax is able to build shield batteries in Co-op Missions. Additional upgrades for them are unlocked when Karax reaches Level 6.

Development
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.

Trivia
The StarCraft II shield battery model's central orb is a miniaturized version of the Purifier technology container from the Legacy of the Void campaign mission Forbidden Weapon.