There are companies that will take the packs apart and rebuild them with new cells online- you can search around. There is no magic science or software to lithium's however they do handle different than NiCD and NiMH as mentioned above.
There are ta few challenges with trying to rebuild a liIon pack at home:
1- First you have to determine what type of cells. Is it Lithium Cobalt (typical laptop cell)? Lithium Cobalt polymer? Lithium Managese? Lithium Iron Phosphate (LIFePO4, A123 Systems)? there are about 3 or 4 others...
2 - Loose Lithium cells are not nearly as easy to obtain as loose NiCD/NiMH cells (can buy from any hobby supplier)
3 - The packs were factory assembled with spot welded metal tab connections. At home you will have to solder the cells. If you are a novice to soldering this can be tricky as you want a good solid bond while avoiding damaging the cells by getting them too hot.
To add to the above responses, some general comments about the different battery characteristics:
- Unlike NiCD (and to a lesser extent NiMH) Lithium Ion do not tolerate ANY overcharge. You can not just slow charge them all the time, the charging devices for LI tools are always smart CC/CV chargers that sense the max voltage and stop
- Unlike NiCD, packs of lithium will not self balance among the cells while charging. For this reason all lithium chargers sense the voltage of each cell independently and use techniques to vary the current and balance cell by cell (that why the packs sometimes have 6/8/10 pins on the connector). In either pack type replacing one bad cell in a brand new pack can work, but replacing once cell in an older pack can be problematic due to mismatch under charging.
-NiCD are tolerant of being stored at pretty much any state of charge. Lithium Ion last the longest when stored around half charge, and will have severely shortened life when stored full. Some devices try to extend their life by only charging to 80% full and limiting discharge depth to 20% full. ( LiFePo4 is an exception to this rule)
- Lithium Ion lifespans in both cycle life and calendar life are much lower than NiCD. Typical specs are 300-500 charge cycles depending on usage and only a year or two of calendar life before significant capacity loss is evident. I suspect a lot of power tools use the 80% max charge trick to achieve these 3+ year warranty lives.
If you want to learn all about batteries, the
http://batteryuniversity.com/ website is a good starting point.