Overall nickel metal hydride batteries

Nickel-metal hydride batteries have become the dominant advanced battery technology for EV and HEV applications by having the best overall performance to meet the requirements set by USABC. In addition to the essential performance targets of energy, power, cycle life and operating temperature, the following features of NiMH - have established this technology in this market ... 

Lead-acid, nickel-iron (Ni-Fe), nickel-cadmium (NiCd), and nickel-metal hydride (NiMH) batteries are the most important examples of batteries with aqueous electrolytes. In lead-acid batteries, the overall electrochemical reaction upon discharge consists of a comproportionation of Pb° and Pb4+ to Pb2+. All nickel-containing battery reactions are based on the same cathodic reduction of Ni3+ to Ni2+, but utilize different anodic reactions providing the electrons. Owing to toxicity and environmental concerns, the formerly widely used Cd°/Cd2+ couple (NiCd cells) has been almost entirely replaced by H/H+, with the hydrogen being stored in a special intermetallic compound (NiMH). 

The mass of lithium ion batteries allows the automotive chemist and design engineers to decrease the mass of the overall vehicle as well as increase the performance of the vehicle. They are much lighter than other rechargeable battery designs. Lithium ions are also a wise choice for automotive use because self-discharge is very low (5% per month vs. 30% for nickel-metal hydride [NiMH] batteries). 

Long-term storage at high temperatures, similar to operation at elevated temperatures, may deteriorate seals and separators and could cause permanent damage, such as loss of capacity, cycle life, and overall battery lifetime. The recommended temperature range for long-term storage of nickel-metal hydride cells is 20 to 30°C. 

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