Cell voltage nickel metal hydride

Numerous other types of cells exist such as zinc-air, aluminum-air, sodium sulfur, and nickel-metal hydride (NiMH). Companies are on a continual quest to develop cells for better batteries for a wide range of applications. Each battery must be evaluated with respect to its intended use and such factors as size, cost, safety, shelf-life, charging characteristics, and voltage. As the twenty-first century unfolds, cells seem to be playing an ever-increasing role in society. Much of this is due to advances in the consumer electronics and the computer industry, but there have also been demands in numerous other areas. These include battery-powered tools, remote data collection, transportation (electric vehicles), and medicine. 

However, concerns about the toxicity of cadmium have accelerated the replacement of these batteries by nickel-metal hydride batteries, described in Section 9.3.5. In nickel-cadmium (nicad) batteries, the anode is cadmium and the cathode is an unstable nickel oxyhydroxide, formed in the unusual conditions found in the cell, and written variously as Ni(OH)3 or NiO(OH). It is generally formed together with stable nickel hydroxide, Ni(OH)2. The electrolyte is NaOH or KOH. The anode and cathode are assembled in a roll separated by a cellulose separator containing the electrolyte. The cathode/separator/anode roll is contained in a nickel-plated stainless steel can (Figure 9.10). The cell voltage is 1.3 V but the working voltage is usually nearer to 1.2 V. The schematic cell reactions are as follows. 

SECTION 20.7 A battery is a self-contained electrochemical power source that contains one or more voltaic cells. Batteries are based on a variety of different redox reactions. Several common batteries were discussed. The lead-acid battery, the nickel-cadmium battery, the nickel-metal-hydride battery, and the lithium-ion battery are examples of rechargeable batteries. The common alkaline dry cell is not rechargeable. Fuel cells are voltaic cells that utilize redox reactions in which reactants such as H2 have to be continuously supplied to the cell to generate voltage. 

Various types of plastic materials are used in the different systems. In lead-acid batteries it is a must to use glass, rubber, or plastics on account of the high cell voltage that would destroy all metals. The advantage of a plastic container is that no insulation is required between adjacent cells. A general drawback of plastic materials is their permeability for gasses, water vapor, and volatile substances. Therefore, with sealed nickel/cadmium batteries and also nickel/metal hydride batteries metal is used as container material. 

The characteristics of nickel/metal hydride batteries are very similar to those of sealed nickel/cadmium batteries. The cell voltage differs by only 20 mV, and charging as well as discharging performance are so alike that both battery systems can be replaced by each other in all normal applications. The discharge curves in Fig. 1.38 confirm this. 

Figure 1.38 Comparison of discharge voltage and capacity between nickel/cadmium and nickel/metal hydride batteries of the same size. Cylindrical cells, AA type, discharging current 0.2CA ( 5 hours) (from Ref. 74).

In battery systems based on aqueous electrolyte, water decomposition, which occurs above a cell voltage of 1.23 V, is such an unavoidable secondary reaction. But under certain conditions the resulting water loss can be avoided, and the system is used as a sealed one, as achieved with sealed nickel/cadmium, nickel/hydrogen, and nickel/metal hydride batteries. In lead-acid batteries corrosion is an additional unwanted secondary reaction with the consequence that lead-acid batteries cannot be made virtually sealed, but must have a valve, and a certain water loss cannot be prevented. 

FIGURE 29.7 (a) Discharge capacity vs. ambient temperature for sealed cylindrical nickel-metal hydride batteries at various discharge rates end voltage 1.0 V/cell. (b) Discharge capacity % of 0.2C rate) vs. discharge rate (C-rate) for sealed cylindrical nickel-metal hydride batteries at various temperatures end voltage 1.0 V/cell. 

While the memory effect may result in reduced battery performance, the actual voltage depression and capacity loss are only a small fraction of the battery s capacity. Most users may never experience low performance due to this behavior of the sealed nickel-metal hydride cell. Often memory effect is used incorrectly to explain a low battery capacity that should be attributed to other problems, such as inadequate charging, overcharge, or exposure to high temperatures. 

FIGURE 29.17 Cell voltage vs. charge input for sealed cylindrical nickel-metal hydride batteries. (a) At various temperatures (charge rate 0.3C). (b) At various charge rates at 20 U. 

Table 29A(a-d) lists some of the portable sealed nickel-metal hydride cells and batteries that are manufactured and their physical and electrical specifications. Multicell batteries are also manufactured using these cells or batteries in a variety of output voltages and configurations. Manufacturers data should be consulted for specific details on dimensions, ratings, and performance characteristics as they may differ from those shown. ... 

TABLE 29.4a Specifications of Sealed Cylindrical Nickel-Metal Hydride Single-Cell Batteries, Nominal Voltage 1.2 V... 

The intrinsie voltage of the couple is approximately the same as with niekel-czdmium values of 1.2 to 1.35 V per eell having been quoted. This helps in maintaining applications compatibility between existing niekel-eadmium cells and the new nickel-metal hydride eells. 

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