These are brilliant and powerful drills, however,they suffer from having NiCd batteries which not only suffer from the memory effect but are stinky polluting cells.
The good news is that you can replace the cells with high amp versions of the 18650 Li-ion cells, you can get away with 10 cells but to make it run for longer you need 20 cells in pairs so that you have 10s x 2p. There is room for them.
It is a great hobby project and the cost of a new drill will encourage you!
You need a BMS inside the battery back to make sure the cells are charged in a balanced way, this will make them last for many years and unlike NiCd they will not suffer the memory effect.
If you look on youtube you will see many people converting drills to Li-ion 18650. Some say you can used recycled laptop cells but they are not powerful enough for the torque you need on a drill.
However, if you can pick up some old 18650's that were used for drills, mowers, garden tools etc then they would probably be strong enough. Look for such things on Freecycle.
Some people take the time to ad an Amp and Voltmeter so they can see how much charge is in the cells. The good thing about these cells is you do not need to wait till they are used up to charge them.
The BMS will make sure that the cells are only charged as needed and protect from overcharge or overdrain. You will find additional help on 18650 forums if you get stuck
Mike, October 2019
