Warning: The following text contains spoilers relating to the plot of Prophecies.
Soul Batteries are the magical devices that hold the Door of Komalie shut. They are "powered" by the souls of the Chosen that the White Mantle and Mursaat sacrifice upon the Bloodstones, hence their name. Soul Batteries look like Mursaat Towers and have seals at their bases exactly like the Mursaat Towers.
In Abaddon's Mouth, your mission is to release the souls powering the Batteries by breaking the seals at their bases. In the next mission, Hell's Precipice, realizing that you have unleashed the Titans upon the world, your goal is to re-power the Soul Batteries by slaying the Lich Lord on the Bloodstone at the Door.