The name NiMH is actually an abbreviation which stands for Nickel-metal hydride battery. It is a type of rechargeable battery. The distinct characteristics of a NiMH in terms of energy density, nominal cell voltage, charge/discharge efficiency, energy density and cell durability are as shown below:
Nominal cell voltage-1.2 volts
Energy Density-140-300 Wh/l
Specific power-250-1000 w/kg
Charge/discharge efficiency-66%(varies with temperature)
Cell durability-500-2000 cycles
The History of NiMH Batteries
The NiMH battery evolved from the nickel hydrogen battery which was used in aerospace applications. Since they have an exceptional cycle life and a favorable specific energy, they were used for aerospace applications for a long time before they were introduced to commercial uses.
Scientists started working on NiMH batteries in 1967 at the Battelle-Geneva Research Center. The basis of the work was scientists discovering that some metal alloys actually had the ability to store atomic hydrogen which is amazingly 1000 times their own volume. The metal hybrids are hybrids which were based on compounds such as LiNi5 or ZrNi2.Development was catalyzed by Daimler-Benz and Volkswagen AG. It was done by Deutsche in Automobilegesellschaft(a Daimler AG subsidiary)At the time, the battery specific energy was 50 W.h/kg while the specific power was up to 1000/kg.The cell durability was 500 charge cycles.
Sample applications were filed in countries in Europe (Mostly in Switzerland), The United states and Japan. They were later taken to Daimler Beinz and further to other parts of the world.
In the 1970s nickel-hydrogen used for satellite applications was commercialized thus interest in NiMH grew rapidly. Since the hybrid technology was not bulky in terms of storing hydrogen, it provided a better alternative. Philips Laboratories and France’s CNRS carried out research and developed new high-energy hybrid alloys which incorporated earth metals. These were used in the negative electrode. Sadly, they suffered from alloy instability in alkaline electrolyte thus causing insufficient cycle life.
In 1987, Buschow and Willems corrected the above error by using a mixture of La0.8 Nd0.2 Ni2.5 Co2.4 Si0.1.The battery now kept 84% charge capacity even after 4000 charge-discharge cycles which was a massive improvement. Viable alloys which used mischmetal in place of lanthanum were later developed. They were more economical.NiMH being used today are based on this design.
The first NiMH Cells were released to consumers in 1989.However, Ovonic Battery Company which was based in Michigan changed and improved the structure of Ti-Ni alloy and composition. The changes made included inclusion of special alloys which had a disordered alloy structure. Specific multi component alloys were also added. Unfortunately, the cycle life of the alloys remained low due to their composition.
For household sizes until the late 1980’s, the NiCd’s were the only available option for rechargeable batteries .However, their capacity was not good and they also contained toxic cadmium thus were supposed to be disposed as waste and not in household pits for health purposes. The NiCd’s were replaced by the NiMH which offered triple the capacity of NiCd’s and contained materials which were not hazardous. They both costed around the same price. As a result, NiMH became popular and the NiCd’s disappeared.
A high energy electrode which was developed by Dr.Masahiko Octane (GS Yuasa Company) led to the NiMH cell.The technology was further used in 2008 as more than 2 million hybrid vehicles were manufactured with NiMH batteries worldwide. At this stage the batteries were advanced hence their use in hybrid vehicles. The batteries were more portable for consumer use. Due to this factor, the European Union recommended that Ni-CD be replaced by the more effective and efficient nickel-metal hybride batteries. Statistics in Japan show that approximately 22% of portable rechargeable batteries were sold in 2010.As we can see, the batteries were proving to be efficient and effective. On the other side in Switzerland 60% were sold in 2009.Currently, Lithium-ion batteries are replacing the NiMH batteries though the NiMH batteries are still very popular.
In 2015. Germany’s BASF has made a step forward by producing a modified micro-structure which has helped to make NiMH batteries last longer. This has allowed changes to the design of the cell thus saving weight.It has also helped to increase the gravimetric energy density to 140 watt hours per kilogram. The NiMH battery technology is pervasive today and powers everything from hybrid electric vehicles to cellular phones, clocks, digital cameras etc.