Power sources , or power cells , were First Civilization artifacts that powered the Grand Temple. In , Desmond and William Miles had to retrieve three power sources located in different cities around the world. For the first one, Desmond traveled to New York City , climbing One World Trade Center and parachuting onto the building that contained the power source. After he retrieved it, he was confronted by the Master Templar Daniel Cross , whom he knocked out with the artifact.
The Battery is a device that first shows up in Assassin's Creed 3 inside the temple. It is a cube like power source used to power up the door and they are a necessary component to have before you are able to use the key. You gain the first Battery inside the temple that just happens to be next to the port it needs to be placed into. The second and third battery show up later in the game and requires Desmond to go out of the Temple and retrieve them himself but along the way he runs into Daniel Cross both times. The Fourth and final Battery is first sought after by William Miles who goes after it to make sure Desmond has more time to find the Necklace and gets captured as a result forcing Desmond to leave anyways and using the Apple retrieves his father and the final battery.