updated
When can you get it: During or after the main story mission Maker's End
The third power cell can be found in Maker’s End, initially during the story quest of the same name.
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Once you’ve scaled the F.A.S. building all the way up to Faro’s Office - near the end of the mission - you’ll want to scale the tower at the east end of the office to the very tip-top of the skyscraper.
Pick up the power cell, which can be found by the edge of the tower, and then proceed to rappel back down.
