Plasma battery

The Covenant plasma battery in Halo: Reach.

The plasma battery is the Covenant equivalent to the UNSC fusion core and the Forerunner power core. The exact purpose of it is unknown, though it probably holds some sort of energy needed for Covenant vehicles, machines, and equipment. However, they can be used as improvised explosive devices, due to their explosive reaction to weapons fire. They require more shots than the fusion coil to explode. They also require a higher drop to detonate them on impact. It is possible they are a Covenant power source for plasma weapons which recharges them when they run out of battery power.

They appear in both campaign and multiplayer, and can be very useful to whomever sees the right moment to detonate it, such as near a pack of Grunts. These can also be bought when in Forge, costing a fair price. They can be placed anywhere on the map, even in mid-air. If in a last-ditch situation, you can kill enemies standing near either a plasma battery, a power core or a fusion coil by shooting at it. Any enemy killed using the explosion from a plasma battery will be listed as killed by you using an "Unknown" weapon. It appears to have energy shields as part of its structure, however, you don't need to deplete them to detonate it. There are stacks of them on the level, The Ark, and can be used to kill nearby enemies. Once one explodes, it causes a chain reaction of the explosions of nearby plasma batteries.

Trivia

It takes two explosions from a Plasma Battery to kill a fully shielded SPARTAN-II or Sangheili.
Plasma Batteries are most likely used to recharge Covenant weapons that utilize batteries, such as the Plasma Rifle. However, this is impossible in-game.