-( 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: Sentinel Beach
While running on the cliffs collecting Precursor Orbs and Power Cells (as well as Scout Flies), you will probably come across a large egg. Personally, I forgot to talk to the old woman below the egg, but it isn't necessary. Ram/punch the egg off the cliff and it should land next to her. She will give you a Power Cell and Daxter will go through motherhood.