Get the power cell over the lake was a mission that Jak and Daxter completed to obtain a power cell in The Precursor Legacy. Jak collected this lone power cell that was spotted high up above the lake.
There is a power cell sitting over the lake in the very center of the second area in the Precursor Basin. To get this one, find the path leading to a raised donut-shaped area. From here follow the thin path in front of you to the next platform. Here you need to make a small jump to yet another platform followed by another jump off a ramp to a larger section next to a cliff.
Turn slowly right and then left as you go up the path, taking care to avoid any dark eco boxes. At the top you'll have to jump across a series of pillars leading downwards, going too slow will cause you to fail so make sure to not slow down. You'll then land at another platform (together with a locked box), on your left is the final ramp marked by a trail of Precursor orbs. Charge into them and jump off the ramp to reach the power cell.