The last 3 Power Cells are locked in story missions. If you miss them during the mission you can go back, but you won't be able to access the area until you reach that point in the story. The last one isn't available until one of the last missions so I was already around 30+ when I got it, but I hadn't done much side content leading up to that so I'm sure that number would be higher. I finished the game at 37, and got 3 levels after the point of no return.
When I pick it back up to hit up some of the side quests and future DLC it will be nice to have the armor in hand. The last few boss battles drained my armor a ton, so I imagine without the armor I would have had to play a lot more cautiously.