It is often stated that NiCd batteries must not be totally discharged. This is not entirely true but in practice, it is difficult to prevent polarity reversal of one or more cells.
The polarity of a cell in a battery that gets discharged first will be reversed by the other cells in series with it. This is usually the end of the cell because its internal structure cannot cope with this condition.Manufacturers try to prevent this by making the positive electrode of battery cells more robust. But even then, the cell will be damaged by regular polarity reversal, particularly at high charging currents. Once a cell is damaged, its capacity is reduced, which causes it to get discharged more quickly, …, and a vicious circle is set up. A possible solution to this problem is to shunt each cell with a Schottky diode that can handle a current equal to, or larger than, the charging current.