NiCd cells are usually pretty well behaved, they have to get *really* old before they start to leak. In most cases NiMH cell characteristics match closely enough that they can be used without major consequences. Li-based cells are definitely out, they have drastically different characteristics, roughly 3x the cell voltage and require far more intelligence in the charger. Not to mention they don't take kindly to misuse, in other words they like to explode into flames.

There are many shops that rebuild battery packs for power tools and such that should be able to rebuild a Powerbook pack easily enough. If I ever have time, I've got a 520c with dead batteries that I plan to tackle. A few years ago I built a capacitive discharge spot welder for welding the nickle strip to the cells. Without that you will need cells with tabs on them, *don't* try soldering directly to the cells.