Energy cells are small, self-contained energy storage units often associated with one-handed energy weapons and act essentially as a very powerful battery or capacitor. They are the standard ammunition used by the majority of pistol energy weapons, and as such are widely available across the Mojave Wasteland.
Bulk energy cells are cheaper than the regular variant, but not as potent. Weapons degrade more slowly using this ammunition, however damage is reduced and there is no armor-piercing effect.
Energy cell, over charge
This ammunition is more powerful than standard energy cells. It increases the damage output of weapons by 1.25 and has a -5 DT effect on enemies, but degrades weapons 1.5 times faster than normal.
Energy cell, max charge
This is the most powerful of the energy cell variants. Max charge cells increase damage output to 1.5 times normal and have an armor piercing effect of -10 DT, but cause weapons to degrade 2.5 times faster.
Energy cell, optimized
Added in Gun Runners' Arsenal, this sub-type causes 30% more damage, penetrates armor more effectively, and weighs less than the standard energy cell. The only drawback is a slight increase in weapon degradation. It requires Vigilant Recycler and can only be crafted at a workbench. This type of cell is slightly superior to overcharged cells.
Hidden Valley bunker - If you gain accepted status or higher with the Brotherhood of Steel, you will be approached by a BoS initiate telling you they have placed recycled ammunition in a box at the bottom of the entrance stairs to the L1 area. The box will be replenished randomly with energy cells, microfusion cells and electron charge packs whenever you enter L1, giving a continuous supply of energy weapons ammunition.
Extremely large numbers can be found in one of the boxes that appears during Caesar's Favor and Caesar's Hire (sometimes over 1800 can be found). The box that the cells appear in only appears once you reach Idolized reputation with Caesar's Legion.
Old World Blues (add-on) - Muggy can provide 50 a day once given the "Parts Production" upgrade.
Old World Blues (add-on) - Once you have upgraded the sonic emitter, you can go to Doctor 8 and he will give you 100 regular, overcharged and max charged energy cells.
Despite their name, energy cells are abbreviated as "SEC" (small energy cell) at workbenches. Additionally, the drained cell sometimes left behind and the bulk ammunition box also adopts this trait, being called small "drained energy cell" and "Ammo box, small energy cell, bulk" respectively.