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 bugged. It has the same name as normal Tesla Armor and the same stats as Enclave Power Armor. When considered logically, 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, as well as the fact the electricity effect you get with it floats around your character without coming from anywhere. It's possibly something the developers forgot to disable or remove.

Location Theory
Across several online Fallout 3 forums, people have been debating back and forth 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 inaccessible by the player raining down fire from above.(It is accessible in both 360 and PC versions, through the stacking of barrels) It is thought that killing this soldier, as well as his comrade also positioned next to him, might reveal the Backpack. It is also possible, through the physics engine, to kill him and fling his body over the edge. A 360 user was able to do this, but did not find the Tesla Power Pack. The Enclave soldier does drop Tesla Armor and a Tesla Helmet and a Plasma Rifle(and MF Cells), but it is NOT the Tesla Power Pack. This information is confirmed for both PC and 360 versions of the game.