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Nickel-Metal Hydride Rechargeable Battery (Glossary) |
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| Nickel-metal hydride rechargeable batteries are the technical further development of the Nickel-cadmium type. their performance data is greatly improved, only the resistance to cold and the power density, in other words the maximum discharge currents per unit of weight, cannot quite compete with those of its predecessor. Decisive for their rapid success on the market as successors to the Ni-Cd type was the fact that with the same terminal voltage and assembly, they are nonetheless free from cadmium, and thus much more environment-friendly. In nickel-metal hydride secondary cells with a basic electrolyte (generallycaustic potash solution), hydrogen is oxidised to water at the anode and at the nickel cathode, trivalent nickel oxide hydrate is simultaneously reduced to its bivalent form. The anode consists of an alloy of nickel and rare earths (lanthanoids, i.e. cerium, lanthanum, neodymium, praseodym, for example) and only serves as a hydrogen store. The overall reaction, greatly simplified, is as follows (La=lanthanoid):  Typical performance data and uses  Cell terminal voltage: 1.2 - 1.3 volts Service life in charge cycles: 1,000 to 1,500 (2,000) For use in ambient temperatures: -15°C to +40°C Typical energy density: 50 - 60 (70) Wh/kg (theoretically approx. 300 Wh/kg) Typical volumetric energy density: 170 - 250 Wh/l Typical power density: 100 - 200 W/kg Typical spontaneous discharge: approx. 20-50% within 1 monthA disadvantage is the relatively high spontaneous discharge and the relatively high cost of production compared to nickel-cadmium accumulators. NiCd cells have extremely diverse fields of use: millions of individual small cells can be found in a wide variety of devices such as torches, portable phones or CD players. Assembled into battery packs (Power-Packs) they are used as energy sources in mobile phones, notebooks or cordless screwdrivers. Historical development The nickel-cadmium accumulator developed in 1899 was the first system that was able to establish itself on a large scale after the earlier lead accumulators. Its ratings were excellent, but the market demanded a replacement of the toxic cadmium. Thus, the first nickel-metal hydride cells were developed in 1970, but failed on account of the at that time still unstable mixed metal alloys. Significant improvements were made around 1980 and the first marketable models appeared around 1990. Since then, nickel-metal hydride accumulators have been steadily ousting their nickel-cadmium predecessors from the market. © Marc Stenzel |
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