-( Story related, cannot be missed )-
*While this is "flagged" as possibly story related, the real note is that... you could scrape by without having all 101 Power Cells early on. It would be in your best interest to avoid back-tracking at the end to get this for the final ending.
Level: Forbidden Jungle
Near the back door to the Precursor Temple in Forbidden Jungle, you will find a pumping station. Climb to the top of it and you will find that the Blue Eco beam mentioned by the Mayor of Sandover is being redirected. So, it is up to Jak and Daxter to fix what the Lurkers have done to the beam. Break the mirror, then redirect the beam.
Once you activate the controls, you will see through the HUD an aiming reticle with (possibly) an arrow. The arrow is pointing you to the next mirror. Aim at it and then proceed to where it was to aim from there. You will have to hop all across the jungle to finish this little task. Once you have connected the final beam, you will be able to collect the Power Cell from the Mayor at any time.