Alien Power Cell (Fallout 3)

The alien power cell is an energy weapons ammunition of alien origin found in Fallout 3.

Characteristics
It is used with two weapons, the alien blaster and the Firelance. In previous games, alien weapons simply used small energy cells.

Used in

 * Alien blaster
 * Firelance

Locations
A maximum of 1152 alien power cells can be found in all expansions, without the raider glitch.

Base game
A maximum of 362 alien power cells can be found in the core game.
 * 24 can be found together with the Firelance in the random encounter "Unidentified Flying Debris".
 * 120 (10 stacks of 12 cells) can be found in and around the Alien Crash Site.
 * 96 (8 stacks of 12 cells) can be found under an overturned truck in a radioactive crater due east of Fort Bannister. When you see bombed out buildings you may want to adjust your heading a little to the north so as to come right into it. There may be some Enclave at the site and an Enclave scientist at the truck checking it out (Note: You will only suffer minor radiation, approximately 1 Rad/sec).
 * 2-22 without the Scrounger perk (17 on average), or 2-122 with the perk (92 on average), can be found in two ammo boxes in Fort Independence, in the Outcast weapons room (hard locked door) where you find the Miss Launcher. The method by which the outcome is determined (rolling for 1s at a 75% chance, 10 and/or 50 times) pretty much guarantees that the results will be on the high side (i.e., near 75% of 10 or 50). See Talk, for more info.
 * 10 or so on the eastern edge of the map, scattered in a gully south of vault 87.
 * It is also possible to get alien power cells by finding a group of raiders (it doesn't necessarily have to be raiders, but the glitch works best with them), killing one, and placing the alien blaster and 1 power cell on his body. You then break the other raiders' weapons, and once one of them picks up the blaster, kill him and look at his inventory or sneaking up on one, pickpocketing his weapons and ammo, then replacing them with the weapon and ammo. He should appear to only have one, but take it back anyway, and look at your alien ammo. There should be more: usually around 10.

Broken Steel add on
A maximum of 425 more can be found in Broken Steel.
 * In Broken Steel during the Shock Value quest, they can be found in two separate locations within Olney Powerworks. In the room with the automated turrets and walls of computers, just above the turrets there are two ammunition boxes each with an average of 40 to 45 (Scrounger-affected esteem). Near the Tesla coil, in the room at the bottom of the coil's pit, there are 36 (3x12) power cells on the stove and table.
 * A nice amount can be found in the Armory of the Mobile Enclave Crawler at Adams Air Force Base along with an alien blaster: from 10 to 12 are inside the alien blaster case in the armory (this particular stash is NOT affected by the Scrounger perk). Plus, inside the 3 average-locked ammo crates just below the blaster's, you will find an average of 46 cells each, pretty much like the containers in Fort Independence's storage room, assuming that you have the Scrounger perk; otherwise, you will not get more than 30-36.
 * If you destroy the Citadel during the "Who Dares Wins" quest, 72 (6 clips of 12) can be found in the armory in a room with mini nukes and Tesla cannons.

Mothership Zeta add on
There are approximately 365 APCs on Mothership Zeta; there may be more, but this is the list so far:
 * 48 (4 x 12) on the back shelf of the room where you reunite with Sally after she guides you through the Steamworks; there is also a healing archway and two containers in the same room.
 * 44 (2 x 12, 1 x 20) in the Robot Assembly area, downstairs in the first chamber, on a corner shelf near a teleporter room, near a small container, opposite the robot escalator.
 * 26 (1 x 15, 1 x 11) in a box with a Nuka-Cola Quantum on the second floor of the Engineering Core where Sally and the other NPCs are located. It will only appear after the third generator from Among the Stars is destroyed. The box is located on a railing behind the Workbench; you will have to either jump or pull it over from the side.
 * 51 (3 x 12, 1 x 15) in the Weapons Lab area, on the shelves next to the atomic pulverizer.
 * 87 (6 x 12, 1 x 15) on a set of tables towards the end of the Weapons Lab area near where the destabilizer is found, along with a cache of weapons and armor, including the Gatling laser, plasma mines, and Tesla armor.
 * 109 (7 x 12, 1 x 25) in the Biological Research area on a set of shelves under a catwalk, where you reunite with Sally and the others.

Tips for locating

 * Alien power cells glow in the dark. This can be useful for finding them, since they are small, but they may be scattered randomly, making them hard to spot. Even small dips in the ground or tufts of grass can hide them.
 * Another way to find them is to order Dogmeat to search for ammo when the cells are nearby.