A number of secondary, i.e. rechargeable, alkaline batteries or cells incorporate cadmium as an active electrode material. The most important and best known of these is the nickel-cadmium cell which is based on the reversible electrochemical reactions of cadmium and nickel in a potassium hydroxide (alkaline) electrolyte. At the negative electrode (cathode) the cadmium oxidises to cadmium hydroxide on discharge whilst the hydrated nickel compounds at the positive electrode (anode) are reduced to nickel hydroxide, thus:

2Ni(OH)2+Cd(OH)2 <-- charge/discharge --> 2NiO OH + Cd + 2H20

The potassium hydroxide electrolyte takes no part in the charge-discharge reactions and acts only as a charge carrier. Lithium hydroxide may be added to the electrolyte to increase the life of the positive electrode. The reaction produces a nominal usable electromotive force of 1.2 volts per cell.

A number of types of cell construction are possible. These variations in cell construction lie mostly in the nature of electrode support utilised. For the positive electrode three principal types are recognised - pocket plate, sintered plate and fibre plates. An electrode support is necessary because the active material (nickel hydroxide) is usually in powder form and held in pocket plates or mixed with gel or paste and placed in sintered or fibre electrodes. Also, graphite or iron oxide needs to be added to improve the conductivity of both nickel and cadmium hydroxide.

Negative electrode designs make use of an even broader range of materials including pocket plates, sintered nickel powder, fibre, foam and plastic bonded supports. It is the physical stability of the active material (cadmium hydroxide) in the negative electrode that permits such a wide variety of support materials. Nickel hydroxide, however, swells appreciably during charge and discharge, straining the support and restricting the choice of support type at the positive electrode. In all cell construction types a separator is placed between the two electrodes to prevent short circuits.

Nickel-cadmium batteries are characterised by their resistance to electrical abuse, high cycle lives, reliability and versatility and have found a wide range of application. The several types of cell construction are manufactured in a wide range of size, capacity and shape and the choice of a particular battery will depend upon the application and its current load requirements.

These applications are principally of two types industrial and portable batteries.

(a) Industrial nickel-cadmium batteries

Nickel-cadmium batteries for industrial uses are of the vented (or open) or semi-sealed type and may be of pocket plate, sintered plate or fibre structured construction. Applications for industrial batteries include railway uses such as locomotive starting, emergency braking, coach lighting and air conditioning, trackside power for signalling and warning lights and others. Other uses include standby power for alarm systems, emergency lighting, military communications, solar energy storage, navigation equipment, military equipment, hospital operating theatres and many others. Semi-sealed industrial batteries are used in aeronautical applications where they are used to start engines and also to provide stand-by power for aircraft systems when the principal power source fails. After long periods of operation most vented or semi-sealed cells may require electrolyte maintenance by topping up with distilled water.

(b) Portable nickel-cadmium batteries

Nickel-cadmium batteries for portable use are of the sealed type and are generally of sintered plate construction. They may be of cylindrical , button or prismatic design. Sealed nickel-cadmium batteries are in strong demand for use in consumer electronic equipment such as cellular telephones, portable tools, toys, camcorders and other domestic cordless appliances. They are also used for memory back-up in computing equipment, military and civil communications, emergency lighting and many other similar applications. Sealed cells require no maintenance and may be recharged up to 2000 times.