Nickel safety vent

Figure 2.6 shows the typical structural design of a cylindrical nickel-cadmium battery. It has a safety vent, as illustrated in Figure 2.7, which automatically opens and releases excessive pressure when the internal gas pressure increases. Formation of hydrogen is avoided by extra Cd(OH)2 oxygen is removed by reaction with Cd. 

The construction of this cell is similar to that of standard cylindrical nickel-cadmium cells. It is sealed, has a safety vent and makes use of a cylindrical spiral-wound plate design. 

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. 

The D-cell was the first cell available within this range. It is rated at 2.5 Ah at the lOh rate. It is 61 mm in height and 34 min in diameter. The construction of this cell is similar to that of standard cylindrical nickel-cadmium cells. It is sealed, has a safety vent, makes use of a cylindrical spiral-wound plate design for high energy density and low internal impedance, and can be charged and discharged in any orientation. 

Batteries may produce potentially explosive gases, particularly near the end of charge or when over-discharged. Any battery which has a safety relief vent has the potential for venting hydrogen or other explosive gases. The nickel-zinc battery presents no special hazard in this respect compared to other batteries which are capable of venting. 

Another major applieation of vented nickel-cadmium batteries is aircraft batteries. These batteries are equipped with a battery temperature monitoring facility in order to comply with the requirements of the FAA. One or more thermal switches are installed in the battery to give advance warning and to prevent overheating of the battery, thereby achieving a greater safety of flight operation. 

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