When a cordless tool battery dies, it is likely that _some_, but NOT ALL cells have died. Perhaps 1/3rd of the individual cells can still take a charge.
For the Ryobi brand NiCad battery, the individual cells are slightly smaller than, but compatible with C-Cell devices.
Disassemble the plastic case, and cut, or break the connections between cells. This battery just needed a phillip's screwdriver to remove the plastic case, and pliers to break the connections between cells.
In this picture, I've marked the + end of each cell (in red) before putting them in the charger. This will prevent charging them backwards. These cells are like conventional C-Cells, in that the + end has a bit of a bump. It is less pronounced than in most C-Cells. If there is any doubt about polarity (+ and -), check a cell with a multimeter. Some of the cells will likely still have a small charge on them.
If cells are leaking, bag it all up and dispose of it properly. Home Depot, Lowes, and likely almost anyplace that sells these tools will also take them back for recycling. I haven't checked the MSDS on this stuff, but you don't want to keep leaky cells around, and landfill isn't a good option either. Wash your hands and clean your workspace if you've opened a case with leaking cells.
At this time, I do NOT recommend disassembly of _Lithium-Metal-Hydride batteries_. As I understand, if the internal chemistry meets air, it can catch fire.
Step 1: Check the Battery Composition
Regardless of what battery you use, check that the chemistry of the cell matches that of the charger.
Step 2: Set Charger to Match Cell Composition
Different cell chemistries have slightly different voltage needs for charging.
My charger has a switch to select what type of cell you want to charge.
Using the wrong charger might cause the cell to leak or explode. So pay attention. Sometimes chargers have this info in small print.
So- Ni-Cad charger for Ni-Cad cell.
NiMh for NiMh, etc.
This is a project that is environmentally green, since it makes use of a cell which would otherwise be useless. You wont have to buy new cells for your next project, so It is also green in that it can save you money.
Step 3: Cell Holder
Need a C-Cell holder?