Nickel-cadmium cells, operation

Many types of rechargeable batteries are much more portable than a car battery. For example, there is now a rechargeable version of the alkaline battery. Another example, shown in Figure 11.20, is the rechargeable nickel-cadmium (nicad) battery. Figure 11.21 shows a nickel-cadmium cell, which has a potential of about 1.4 V. A typical nicad battery contains three cells in series to produce a suitable voltage for electronic devices. When the cells in a nicad battery operate as galvanic cells, the half-reactions and the overall cell reaction are as follows. 

Despite the fact that cadmium is very toxic and that the number of batteries based on this metal (i.e. nickel-cadmium cells) is very significant, there are very few recycling plants in operation. One very effective recycling process has been designed and developed by SAFT NIFE AB in Sweden to treat both industrial and consumer nickel-cadmium batteries. The flow... 

If for instance an electronic memory bank with a continuous power consumption of 7 pA must be protected for at least 3 months, a battery with a capacity of 2200 hours x 7 pA is necessary. As the voltage is not allowed to drop below 1.2 V during operation, two primary button cells or two nickel/cadmium cells must be prescribed. A lithium cell would also do the job. 

A portable instrument needs to be battery operated, simple in operation and robust in use. We have stripped down a standard 41/min sampling pump (Rotheroe and Mitchell L2SF) and fitted it with associated electronics for fibre precipitation and sensing into a hand-held unit running from rechargeable nickel-cadmium cells. The instrument top block (see Fig. 1) is cast from epoxy resin in which the corona needles, the precipitator electrode, and the detectors are set. The baseplate on which the block seals is made from aluminium. 

The nickel-metal hydride cell is expected to have a service life approaching that of the nickel-cadmium cell. Effects of operating temperature on both discharge performance and charge acceptance are also similar between cells. 

Sealed nickel-cadmium cells normally operate at internal pressures well below the vent pressure because gas evolved during charging is reasily recombined. Compliance with three essential design criteria is necessary to accomplish this ... 

Figure 31.20 Sample capacity versus temperature curve for Eveready OB90 and OB90T sealed nickel-cadmium cells, 1.2 V. 20Ah capacity. Ranges of temperature applicable to operation are cha e, 0-45 C discharge, -20 to 45 C storage, -40 to 60°C. Dischargecurrent 9 mA (Courtesy of Union Carbide)...

The methods of control recommended arc suited to the operating temperature range and the ability of the nickel-cadmium cells or battery to accept or otherwise a degree of overcharge at high rate. 

The reference voltage for the discriminator is matched to the temperature coefficient of the cells that is, —4mV/cell per "C. The Varta TSL system provides a safe and reliable method of fast charging sealed nickel-cadmium cells and batteries. Because of cell tolerances and variations in operating conditions, Varta do not recommend the use of more than ten cells per discriminator. For electrolyte temperatures up to 50°C it is recommended that the charge rate should not be less than the 10 x l o rate, and for electrolyte temperatures not exceeding 35°Cthe charge rate should not be less than 5 x /iq. Provided these... 

Although one of the most common storage batteries is called the nickel/cadmium system ( NiCad ), correctly written (-)Cd/KOH/NiO(OH)(+), cadmium is not usually applied as a metal to form a battery anode. The same can be said with regard to the silver/cadmium [(-) Cd / KOH / AgO (+)] and the MerCad battery [(-)Cd/KOH/HgO(+)]. The metallic negative in these cases may be formed starting with cadmium hydroxide, incorporated in the pore system of a sintered nickel plate or pressed upon a nickel-plated steel current collector (pocket plates), which is subsequently converted to cadmium metal by electrochemical reduction inside the cell (type AB2C2). This operation is done by the customers when they start the application of these (storage)... 

RAM cells are manufactured and shipped charged and have an initial capacity of about 1.8 Ah for AA-sized cells discharged at 50 mA (in comparison with, say, 2 Ah for an equivalent primary cell). This capacity falls to 1 Ah after storage for 3 years at room temperature. At higher drains, the initial capacity drops to about 0.6 Ah at 400 mA (Fig. 6.14). Cells are designed to operate within a temperature range of 0-65°C. The higher internal resistance of RAM cells limits their maximum continuous output current and also their peak output currents in comparison both with primary cells and with nickel-cadmium and nickel-metal hydride secondary cells. A new cell will have an internal resistance of approximately 0.1 2, but this will rise to 0.25 2 with use. 

At the present time, a large number of spent batteries are disposed of directly into the urban waste stream without proper controls. In addition to the most common systems such as zinc-carbon, alkaline manganese and nickel-cadmium, these now include, at an increasing rate, nickel-metal hydride and lithium cells. Such disposal is of serious concern because of the possible effects of battery components on the environment. Consequently, most countries are now evolving policies for collection and recycling. The majority of lead-acid batteries are recycled, but the number of recycling plants in operation worldwide for other battery systems is still very small due to the unfavourable economic balance of such operations (see Table A3.1). Some of the procedures for the disposal and recycling of battery materials are now briefly described. 

The rechargeable nickel-cadmium (Ni-Cad) batteries are used in a variety of cordless appliances such as telephones, battery operated tools, and portable computers. During discharge, cadmium metal (Cd) acts as the anode, and nickel dioxide (Ni02) as the cathode. Both metals form insoluble hydroxides due to the presence of the potassium hydroxide electrolyte. The cell reaction during discharge is... 

---