The energy cell is a type of ammunition in Fallout 3.
Energy cells in Fallout 3 bear a striking resemblance to 3 regular AA batteries attached to each other.
Energy cells are most commonly found on destroyed robots, but merchants also sell them in large amounts. They are sold for at least 2 caps each.
Weapons using this ammunition
Weapon shots per one cell
Oversized energy cells can be found beside Jennings' corpse outside of the GNR building plaza, just after meeting with Lyons' Pride.