Battery

In the Half-Life series, Batteries are used to give the player Armor Points. Batteries also seem to be used for other purposes, most notably in Half-Life 2.

Half-Life
In Half-Life, Batteries can be found almost everywhere. Based on their abundance, it appears that they might have been used for much more than powering up the HEV Suit.

Batteries give the player a different amount of armor points depending on the skill level. In Easy and Medium, Batteries give the player 15 armor, while in Hard, Batteries only give out 10. This means that the player will have to rely on Chargers for a large increase in armor points.

Half-Life 2
In Half-Life 2, new Batteries have been designed by the Combine to suit their needs, such as for powering City Scanners. However, the Mark V HEV Suit has been redesigned to use the new Battery. Batteries will give the player 15 Armor Points, regardless of difficulty level. This means that the player will still need to depend on HEV Chargers to obtain a large amount of Armor Points.

Unused sound clips from the playable Half-Life 2 Beta indicate that Batteries could be charged up at one time, though how this was done is unknown. In the Beta, the Battery model is exactly like the one in Half-Life, only much smaller.

Team Fortress Classic
In Team Fortress Classic, Batteries can be placed in levels, though they never appear in official levels. They recover 15 Armor Points, but will only raise armor points to 100, even if the class's maximum armor is less or greater than that.