A power cell installed into the Doomseye.
A power cell is a battery that was produced by the ancient Bion race on their home planet. While the race is long gone, these power cells were left behind.
In Metroid Prime: Federation Force, Samus Aran discovers a group of Space Pirates stockpiling these power cells, eliminates them, and alerts the Galactic Federation. The Federation, desiring to analyze the cells in more depth, tasks the Federation Force with retrieving the power cells. There are two carts loaded with cells, only one of which is necessary to complete the mission. The second cart can be retrieved to complete a bonus objective.
However, this retrieval mission is nowhere near easy. The cells are located in a stretch of desert land that is prone to severe electrical storms which are capable of destroying the Federation Force's Mechs and the recovered power cells in seconds. To survive, the Force must utilize shelters throughout the area, or Shield Generators. Further complicating their efforts are surviving Space Pirates. They must bring the carts to the cargo ship at the other end of the land, and then hook up to the Extractor before they leave.
One of these cells is later seen aboard the Doomseye.