This article is about the Combine ball. For the Aperture Science energy ball, see Aperture Science High Energy Pellet.
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Energy Balls are generated by Combine Dark Energy reactor systems and transmitted through the plasma conduits of Citadel power transfer systems. Energy Balls can be contained within specialized dark energy cores, portable long-term storage devices that can later be used in Combine dark energy devices. As well as power generators, dark energy cores can be used by the Pulse Rifle, which can store the energy until released by the rifle's secondary fire.
Energy Balls repel themselves from matter of sufficient mass and density, bouncing off walls, ceilings and floors without losing inertia. However, when they strike matter of low density or mass, they are destroyed, causing them to dissolve into nothing. Gordon's HEV Suit, being plated, may explain why the player will sometimes deflect the orb rather than absorb it upon impact.
Energy Balls are subject to zero-point energy fields, allowing them to be grabbed and manipulated by the Gravity Gun. OSIPR-fired Energy Balls can be caught by the Gravity Gun and fired back at their source. Doing this will earn the "Think Fast!" achievement.
Strangely, the Energy Ball seems to be somewhat attracted towards organic matter. When bounced off a solid object, the Energy Ball is likely to slightly change direction towards the nearest enemy despite the fact that its vector should point elsewhere. The angle of deviation appears to be less than 10° though. Whether this is a physical behavior of the ball or a "help" from the developers to assist the player scoring multiple hits with one ball, is unknown.
 Related Achievements
|Any Half-Life 2 game|
|Kill five enemies with the same Energy Ball.|
|Think Fast! (10G)|
|Kill an Elite Soldier with his own Energy Ball.|
 Behind the scenes
 List of appearances
- Half-Life 2 (First appearance)
- Half-Life 2: Deathmatch (Non-canonical appearance)
- Half-Life 2: Lost Coast (Non-canonical appearance)
- Half-Life 2: Episode One
- Source Particle Benchmark (Game files only) (Non-canonical appearance)
- Half-Life 2: Episode Two