Bring 90 orbs to the miners is the name of four identical missions in The Precursor Legacy that Jak and Daxter completed to obtain four power cells. Gordy and Willard, the two miners, were in need of Precursor orbs for which they parted with some power cells in turn, for a total of four times.
As with earlier trade requests, simply have 90 orbs in your possession to trade them for a power cell. This can then be repeated three more times, for a total of 360 orbs. The miners' cave can be found straight after crossing the second bridge leading away from the Red Sage's house. The miners themselves are at the end of the cave.