Sorry for replying late. For cell replacements check specifications thoroghly to meet the needs of your application. Years ago I played with this without lasting success. A lot of cells you get for RC applications have a nice capacity, allow high discharge current peaks, but do _not_ like high charge current. So you would need some current limiter to prevent the quick charge your PowerBook would l perform usually. Cells for standard applications might have a higher capacity but would be worn off by higher discharging current. I was not succesful with any of both because I did not take care of the specific needs of a given cell type. (After the first charging cycle the machine worked well, but after a few cycles the reworked battery pack reached end of life.)

A set of cells flattens a lot of money and time to rework the battery pack. So do this only when you have appropriate instructions and material, probably in combination with an improved charger circuitry. Otherwise it might be a waste of time and money. You better invest into some new (probably overpriced but hopefully) professionally made battery pack still available.