Tesla power pack

Named Tesla Armor in the game, it is actually a third piece of armor that, in theory, you can wear on top of any other gear you have. It does not look like the standard glowing tubes on a Tesla suit's back, but does cover them up if you wear it with the main Tesla Armor suit. It will also give you invisible Tesla coils in the same locations as if you were wearing standard Tesla Armor and arcing electricity on anything you wear it with. Wearing the pack will also give you floating tubes that travel from the top of the pack to your character's chest. Depending on your outfit, the tubes appear to end just below your collarbone and end up floating unsupported.

This can be equipped with any other piece of body armor to add extra Damage Resistance. However if worn with another piece of body armor then 40 Damage Resistance is halved to 20; still an excellent piece of equipment.

This armor also never seems to degrade.

Glitch Theory
It is believed by many in the community that this item is a test item that was not removed. It shares the same name as normal Tesla Armor and the same stats as Enclave Power Armor. When considered, a backpack which weighs the same as a full suit of armor is quite ridiculous.

Some believe it's not supposed to exist at all and was intended as an NPC item only given the confusing name, strange repair options, and the fact that according to the lists it shouldn't be dropped (or even owned) by NPCs. In addition, the electricity effect you get with it floats around your character without coming from anywhere. It's quite possibly something the developers forgot to disable or remove.

This gains further credence by the fact users report the item cannot be dropped. If dropped, it will vanish entirely from the game. While this provides a method to distinguish "Tesla Armor" (suit) from "Tesla Armor" (backpack), from a game design standpoint no item would be allowed through QC with such a glaring flaw.

Location Theories
Across several online Fallout 3 forums, people have been speculating where this "rare drop" might be located on the Xbox 360 version. One theory included the belief that it was a random drop found inside Super Mutant Behemoths after a successful battle. So far, all that has been found inside these giants is regular Power Armor, B.O.S. Power Armor, and in some instances, Outcast Power Armor.

Another theory brings us all the way back to the Main Quest, when the player is attempting to escape Jefferson Memorial with Doctor Li, Garza and the rest of the gang through the Taft Tunnels. A Tesla Armored soldier is placed on a balcony unreachable by the player, raining down fire from above. (Accessible in both 360 and PC versions through the stacking of barrels, and through the use of console on PC. See Taft Tunnel main article for details.)

It is thought that killing this soldier, as well as his comrade positioned next to him, might reveal the Backpack. Through the physics engine, it is also possible to kill him and fling his body over the edge. Many people have used both methods to reach him, but did not find the Tesla Power Pack. The Enclave soldier does drop Tesla Armor and his weapons as expected, but NOT the Tesla Power Pack. This has been verified on both PS3 and 360 versions of the game.

Yet another theory revolves around the inaccessible door underneath the Balcony. Theory states it may be on a trooper past this obstacle. While occasionally multiple Tesla troopers can be drawn out from this door - if the player is loud - the theory is debunked by the fact the door never "opens" - NPCs merely "fade in" at that point.

In summary, most locations are hearsay and unverifiable. The only "legitimate" source is console commands for PC users.