Nickel-metal hydride batteries, sealed

H. Ogawa, M. Ikoma, H. Kawano, and I. Matsumoto, "Metal Hydride Electrode for High Energy Density Sealed Nickel—Metal Hydride Battery," Proceedings of the 16th International Power Sources Conference, UK, 1988. 

The sealed nickel—metal hydride battery has characteristics very similar to those of the sealed NiCd battery. The main difference is that the NiMH battery uses hydrogen, absorbed in a metal alloy, for the active negative material in place of the cadmium used in the NiCd battery. The NiMH batteries have a higher energy density and are considered more environmentally friendly than the NiCd battery. The sealed NiMH battery, however, does not have the very high rate capability of NiCd battery, and is less tolerant of overcharge. 

In 1990, Sanyo and Matsushita initiated large-scale commercialization of small sealed nickel-metal hydride batteries. They are now joined by Dur-acell, Toshiba and Varta in a consortium which is known as the 3C alliance (camcorders, cellular telephones and computers). Several plants have been commissioned which are each producing 100-200 million cells per annum. It is forecast that nickel-metal hydride may overtake nickel-cadmium before the end of the century. In addition to the 3Cs nickel-metal hydride cells are used for a wide variety of cordless consumer products, communications equipment and other high rate long cycle life applications. 

Refs. [i] Linden D (1994) Sealed nickel-metal hydride batteries. In Linden D (ed) Handbook of batteries, 2nd edn. McGraw-Hill, New York, 33.17-33.24 [ii] Crompton TR (2000) Battery reference book, 3rd edn. Newnes, Oxford, p 1/26, 2/19, 313-314, 416-419,19/4-19/6 and Glossary... 

Yuasa K et al (1991) Cylindrical type sealed nickel-metal hydride battery. Natl Tech Rep 37(1) 44-51... 

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. 

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. 

PORTABLE SEALED NICKEL-METAL HYDRIDE BATTERIES ... 

Most of the operating characteristics of the sealed nickel-metal hydride battery on discharge are similar to those of the nickel-cadmium battery. The sealed nickel-metal hydride battery, however, does not have the very high rate capability of the nickel-cadmium battery. In addition, the behavior of the two systems on charge, particularly on fast charge, is different. The nickel-metal hydride battery is less tolerant of overcharge and requires control of the cutoff of the charge, which may not always be required for nickel-cadmium batteries. 

The advantages and limitations of the sealed nickel-metal hydride battery are summarized in Table 29.1. The main advantage of the nickel-metal hydride battery compared to the nickel-cadmium battery is its higher specific energy and energy density. 

FIGURE 29.2a Construction of a sealed cylindrical nickel-metal hydride battery. Courtesy of Duracell, Inc.)... 

FIGURE 29.2c Construction of a sealed prismatic nickel-metal hydride battery. 

The discharge characteristics of the sealed nickel-metal hydride batteries are very similar to those of the sealed nickel-cadmium battery. Several comparisons are illustrated in Chap. 22. The open-circuit voltage of the batteries of both systems ranges from 1.25 to 1.35 V, the nominal voltage is 1.2 V, and the typical end voltage is 1.0 V. 

FIGURE 29.3 Discharge performance of sealed cylindrical nickel-metal hydride batteries at (a) 20°C (b) 45°C. Curves a—0.2C rate curves b—1C rate curves c—2C rate curves d—3C rate. 

Button Batteries. Typical discharge curves for button-type sealed nickel-metal hydride batteries at room and other temperatures are shown in Figs. 29.4a and 29Ab. 

FIGURE 29.6 Discharge characteristics of 9-volt sealed nickel-metal hydride battery (a) Discharge at 20TI . (b) Discharge at 0.2 C rate to 7 volts. Courtesy of GP Batteries, Inc.)... 

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. 

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