Climb up the cliff
After the previous mission, Open the Precursor door, have Jak jump on the rafts on the water, where after he should jump onto higher ground. Use the Precursor drums here to continue getting higher. Jump over another spike-filled gap, to reach another set of drums and a high stone pillar (you'll have to high jump onto this one). After that jump to the other platforms until you reach the final area. The Power Cell is directly in front of you here.