I used to revive old Ni-Cd batteries using a technique similar to what Cat_driver describes. As I refined my technique I learned more about what I was doing.
Some key points;
It's dangerous. You need to build a blast box to contain the battery during the process and safely direct the energy of a malfunction.
Cadmium is a toxic heavy metal. If a battery does go off, you have problems.
Once I had the information on Cadmium, I no longer had any interest in reviving batteries and I disassembled my set up.