| Nickel Metal Hydride |
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| rechargeable batteries | |
| nimh batteries | |
| metal hydrides | |
| nickel | |
A nickel-metal hydride battery, abbreviated '''NiMH''', is a type of Rechargeable Battery similar to a Nickel-cadmium ( Ni Cd ) battery but has a hydrogen-absorbing Alloy for the Anode instead of Cadmium . Like in NiCd batteries, Nickel is the Cathode . A NiMH battery can have two to three times the capacity of an equivalent size NiCd and the Memory Effect is not as significant. However, compared to the Lithium-ion Battery , the Volumetric Energy Density is lower and self-discharge is higher. Common penlight-size (AA) batteries have nominal capacities ''C'' ranging from 1100 MA·h to 2700 mA·h at 1.2 V, usually rated at 0.2×''C'' rate. Useful Discharge Capacity is an inverse function of the Discharge Rate , but up to around 1×''C'' rate, there is no significant difference. The specific Energy Density for NiMH material is approximately 70 W·h / Kg (250 kJ/kg), with a volumetric Energy Density of about 300 W·h/ L (360 MJ/m³). HISTORY NiMH battery technology was developed at the end of the 1980s and commercialised first by the Matsushita Company . APPLICATIONS Applications of NiMH type batteries includes Hybrid Vehicle s such as the Toyota Prius or Honda Insight / Civic and Consumer Electronics . The NiMH technology will also be used on the Alstom Citadis Low Floor Tram ordered for Nice , France ; as well as the humanoid prototype robot ASIMO designed by Honda . Standard NiMH batteries perform better with moderate Drain devices such as Digital Camera s, Flashlight s, and other consumer electronics. Because NiCd batteries have lower Internal Resistance , they still have the edge in very high current drain applications such as cordless Power Tool s and RC Car s. ELECTROCHEMISTRY The Anode reaction occurring in a NiMH battery is as follows: H2O + M + e− ↔ OH− + M-H The battery is charged in the right direction of this equation and discharged in the left direction. Nickel(II) Hydroxide forms the Cathode . The "metal" in the Anode of a NiMH battery is actually an Intermetallic compound. Many different compounds have been developed for this application, but those in current use fall into two classes. The most common is AB5, where A is a Rare Earth mixture of Lanthanum , Cerium , Neodymium , Praseodymium and B is Nickel , Cobalt , Manganese , and/or Aluminum . Very few batteries use higher-capacity negative material electrodes based on AB2 compounds, where A is titanium and/or Vanadium and B is Zirconium or nickel, modified with Chromium , cobalt, Iron , and/or manganese, due to the reduced life performances {Link without Title} . Any of these compounds serves the same role, reversibly forming a mixture of metal hydride compounds. When Hydrogen Ion s are forced out of the Potassium Hydroxide Electrolyte solution by the Voltage applied during charging, this process prevents them from forming a gas, allowing a low pressure and volume to be maintained. As the battery is discharged, these same Ion s are released to participate in the reverse reaction. NiMH batteries have an Alkaline Electrolyte , usually Potassium Hydroxide . CHARGING The Charging Voltage is 1.4-1.6 V/cell. {Link without Title} Duracell recommends "a maintenance charge of indefinite duration at C/300 rate". A fully charged cell measures 1.35-1.4 V (unloaded), and supplies a nominal average 1.2V during discharge, down to about 1.0V (further discharge may cause permanent damage). |
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