Vented sintered plate cells

Vented sintered plate cells are more expensive to produce than pocket plate cells and are therefore restricted mainly to applications where their mechanical integrity and resistance to shock and acceleration together with their high power density are of importance, e.g. in aircraft, helicopters, military vehicles, city buses, etc. Twenty-four volt batteries are generally used, with capacities in the range 30-60 Ah. 

Vented sintered-plate nickel-cadmium cells, in the discharged state, consist of flat positive nickel hydroxide and negative cadmium hydroxide plates, separated by materials that act as a gas barrier and electrical separator. The electrolyte, normally a 31% potassium hydroxide solution completely covers the plates and separators for this reason vented cells are referred to as flooded cells. ... 

FIGURE 27.1 Constant-current charge voltage of vented sintered-plate nickel-cadmium cell, C/10 charge rate. 

A variety of plate formulations are used in vented, sintered nickel-cadmium cells produced by different manufacturers. The plates differ according to the nature of the substrate, method of sintering, impregnation process, formation and termination techniques. The predominate plate fabrication process used for vented sintered plate over the years has been described by Heischer." There are several reviews on electrode fabrication processes that have been used in flooded vented cells. ... 

The service life (discharge time) of the vented sintered-plate nickel-cadmium cell, normalized to unit weight (kilogram) and size (liter) at various discharge rates at 25°C, is approximated in Figs. 27.12 and 27.13. 

Batteries in standby service can be maintained in a fully charged condition by a float or trickle charge similar to pocket plate batteries. The float voltage for vented sintered-plate batteries is 1.36 to 1.38 V per cell. 

Typical Vented Sintered-Plate Nickel-Cadmium Cells... 

A listing of several typical vented sintered-plate nickel-cadmium cells is given in Table 27.3. The 14-, 22-, and 36-Ah sizes are those typically employed in aircraft batteries. Other cells are available in sizes up to about 350 Ah. The larger cells are generally constructed in steel containers rather than the plastic containers now used for the aircraft-size cells. 

Cell construction is mainly confined to two types, using either pocket plate electrodes (vented cells) or sintered , bonded or fibre plate electrodes (vented and sealed cells). In the former, the active materials are retained within pockets of finely perforated nickel-plated sheet steel which are interlocked to form a plate. Positive and negative plates are then interleaved with insulating spacers placed between them. In sintered plate electrodes, a porous sintered nickel mass is formed and the active materials are distributed within the pores. In sintered plate vented cells, cellulose or other membrane materials are used in combination with a woven nylon separator. In sealed or recombining cells, special nylon separators are used which permit rapid oxygen diffusion through the electrolyte layer. 

Iron-nickel oxide cells are always vented. Tubular/pocket plate electrodes are constructed as described above and are generally housed in nickel-plated steel cases. Cells with sintered plate electrodes have smaller inter-electrode spacings. They use synthetic fibre fabrics as separators, and plastic containers. 

In vented cells the sintered metal is cut to the final plate size and, after conduction tabs are welded, the cell is put together in a similar way to the pocket plate cells except that the separator is a thin sheet of low-resistance, small-pore plastic often with a second layer to prevent oxygen transport. These membranes may be cellophane and a non-woven polyamide respectively. 

Energy densities currently being achieved are 20-30 Wh/kg (tubular plate electrodes) and 40-60Wh/kg (sintered plate electrodes). The positive plate comprises thick sintered nickel plates on a nickel plated substrate. The negative plate comprises a mixture of powdered iron and Fe30a. The electrolyte contains 1.2 to 1.3g/cm potassium hydrioxide containing 1-2% lithium hydroxide. The cells are vented. Synthetic fibres are used for separators. 

Rectangular cells with sintered electrodes, SD series (Varta CD type), are suitable for applications in which a high rate of discharge, or operations at extremes of temperature, or permanent trickle is required. The cells are constructed from cut sintered positive and negative plates, separated by a highly porous separator which absorbs all the free electrolyte within the cell. A safety vent is incorporated within each cell to enable gas which may build up under fault conditions to be released. All cells in this range have the cases connected to the positive electrode. All cases are not insulated but intercell separators and nickel-plated connective links are available. The CD series is available made up in metal boxes. 

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