Sealed nickel-metal hydride

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. 

The sealed nickel-metal hydride cell (more consistently metal hydride-nickel oxide cell) has a similar chemistry to the longer-established hydro-gen-nickel oxide cell considered in Chapter 9. In most respects (including OCV and performance characteristics), it is very similar to the sealed nickel-cadmium cell, but with hydrogen absorbed in a metal alloy as the active negative material in place of cadmium. The replacement of cadmium not only increases the energy density, but also produces a more environmentally friendly power source with less severe disposal problems. The nickel-metal hydride cell, however, has lower rate capability, poorer charge retention and is less tolerant of overcharge than the nickel-cadmium cell. 

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... 

In addition to the above general requirements each battery type may have other specific requirements essential for good performance and/or safety. One t5q>ical example is that the separator used in sealed Nickel Metal Hydride (NiMET) batteries should be permeable to gas molecules for overcharge protection. 

Fig. 13.2 Cell balance in sealed nickel metal hydride cells...

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

The car comes with a 330 V, sealed nickel-metal-hydride (Ni-MH) rechargeable battery pack. 

Figure 22.4 is a Ragone plot on a semi-log scale comparing the performance of the nickel-cadmium and sealed nickel-metal hydride in AA size and the new lithium ion battery in a 14500 cylindrical configuration, on a gravimetric and volumetric basis at 20°C. 

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 29.1 Schematic representation of electrodes of sealed nickel-metal hydride cell, divided into useful capacity, charge reserve, and discharge reserve. (Courtesy of Duracell, Inc.)... 

A key component of the sealed nickel-metal hydride cell is the hydrogen storage metal alloy. The composition of the alloy is formulated to obtain a materi that is stable over a large number of charge-discharge cycles. Other important properties of the alloy include ... 

Sealed nickel-metal hydride cells and batteries are constructed in cylindrical, button, and prismatic configurations, similar to those used for the sealed nickel-cadmium 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. 

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.)... 

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. 

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