The power cells cannot be dropped, sold, or "lost". Following the Ancient Armory questline, all power cells can be obtained, but they must all be obtained before you can unlock the armor; which only becomes available in the end-game.
Note: one is "missable"; this is part of a main quest line and while it is hard to miss, if you don't grab it when you are there, the area is inaccessible afterwards.
The third cell is in the Maker's End ruin, which you will visit as part of the Maker's End main quest. Climb all the way to the very top of the ruins to Ted Faro's office. Once there, instead of following the quest objective and examining the data storage system, turn around and find the yellow handholds leading up the ruined sky scraper. Follow these all the way to the top and find the Power Cell there. Use the rappel point to return to Ted Faro's office and continue the quest.
The rest of the cells can be collected at any point; you just need to progress the story line to unlock the areas they can be found if first.