OK, so whats so tricky?
The easy part of charging is reconverting the spent material on the plates to the charged condition. The hard part is knowing when to stop. Let us take a moment to think about what happens when we overcharge the battery. Once all the nickelous hydroxide is converted into nickelic hydroxide, and in theory all the cadmium hydroxide is converted into cadmium, the charging current has to go somewhere. As the energy of the charging current cannot go into more chemical energy, it goes into splitting water (water is still the major constituent of the electrolyte). Just like the age old chemistry experiment of splitting water into hydrogen and oxygen, a fully charged NiCd cell does the same thing. You are forcing oxidation at the positive terminal and reduction at the negative. When one oxidizes water (actually the OH-) ion, one produces oxygen. Likewise, at the negative terminal (now the cathode), one produces hydrogen. This of course is bad. Oxygen + hydrogen = BOOM.
