Alien Power Cell (Fallout 3)

The Alien Power Cell is an energy weapons ammunition found in Fallout 3 of alien origin. It is used with two weapons, the Alien Blaster and the Firelance. In previous games, alien weapons simply used Small Energy Cells.

Locations

 * 24 can be found together with the Firelance in a random encounter, which takes the form of a greenish explosion overhead. NPCs around encounter area can also pick them up.
 * 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.
 * 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). For each box, the number found is:
 * 1 guaranteed
 * Up to 10 more, as a 75% chance of getting one, sampled 10 times. This means you'll almost always get at least 5, and usually 7 or 8, with a max of 10.
 * Up to 50 more, again at 75% chance, if you have the Scrounger perk. You'll usually get around 37 or 38, and extremely rarely more than 43 or less than 32 (but 0 is theoretically possible). For more info, see Talk.
 * 10 - 20 Can be found around area 1-1 on the map near the train to The Pitt
 * In Broken Steel more can be found inside 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 46 (Scrounger-affected esteem).  36 more (this number will not change) can be found in the small kitchen near the tesla coil.
 * A nice amount can be found in the Armory of the Mobile Enclave Crawler at Adams Air Force Base along with an Alien Blaster in the Broken Steel DLC: from 10 to 12 are inside the Alien Blaster case in the armory (this particular stash is NOT affected py the Scrounger perk). Plus, inside the 3 medium-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 above mentioned perk; otherwise, you will not get more than 30-36.
 * During the Shock Value quest of the DLC Broken Steel, as you near the core of Olney Powerworks and the Tesla Coil there are around three or four power cells laying on the Tesla Stoves and tables in the room directly before the Tesla Coil.

More info on the ones in Fort Independence: If you go to Fort Independence, and go in the actual building, go to Fort Independence Lower Level. There should be a tipped over Nuka Machine on the right side of the door. Once you go through that door, you will go down some stairs and come across a door with a terminal on the right. Note that the outcasts inside will be hostile unless you have gained enough points in the The Outcast Collection Agent unmarked quest. You will also need either a lockpick skill of 75, or the Fort Independence key pickpocketed from Protector Casdin. Anyway, after you get through door, go to the bottom floor and go to the southern-most side of the facility. This will bring you to a hallway where, to the right is a door that leads to a small research room filled with weapons data. Behind a locked (Hard) door at the south end of the room is an armory with some ammo and a Minigun. On the table on the left there are two ammo crates. In those crates are the Alien Power Cells.

If you blow up the Citadel, you get around 60 alien power cells by searching the armory (Verified on Xbox 360).

The Alien Power Cell total found in the game seems to be around 550-580 in a game with an average Luck and the Scrounger perk: this number IS limited, and there is no known way (except the console commands for PC users) to get more cells outside the above listed locations.

It's also possible that there will be more Alien Power Cell and possibly Alien Blaster in the 5th DLC, Mothership Zeta.

Finding Them

 * 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.
 * On the PC you can add any amount you want to your inventory using the console command  (to add 10, or any other number).