Description:

Introduction: Nickel metal hydride (Ni-MH) batteries consist of a positive plate containing nickel hydroxide as its principal active material, a negative plate mainly composed of hydrogen-absorbing alloys, a separator made of fine fibers, an alkaline electrolyte, a metal case and a sealing plate provided with a self-resealing safety vent.

Ni-MH has become one of the most readily available rechargeable batteries for consumer use. The rising demand for electric vehicles brightens the market prospects as Ni-MH batteries appear to be the technology of choice for applications such as HEV, emerging EVs, and fuel cell EVs. Nickel-metal hydride battery possesses high energy density and can be charged rapidly by using specially designed charge control system. These features are very useful in portable electronic devices. The exponential consumption of electronic goods and strong supporting policies to expand electric vehicle fleets across the world set to bolster the growth of the Ni-MH battery market.

With the advances of Ni-MH batteries over the last decades, prototype Ni-MH batteries of specific energy over 100 Wh/kg have been established, while the specific power has advanced from 150 to over 1000 W/kg. In addition to the essential performance targets like energy, power, cycle life, and operating temperatures; flexible vehicle packaging, easy application to series and series/parallel strings, safety, maintenance-free operation, fast and less expensive charging, and environmentally acceptable and recyclable materials have established the eminence of Ni-MH batteries.

Apart from being capital intensive, concern areas include high self-discharge rates, low-temperature operation, and higher cooling requirements. Limited service life of the battery due to self deep discharge and highly capital intensive may hinder the growth of the market. The performance of the battery deteriorated if stored at an elevated temperature set to restraint the adoption of the battery in heavy applications. 

Maturity Timeline: Work on Ni-MH batteries began at the Battelle-Geneva Research Center following the technology's invention in 1967. 

Ni-MH batteries suitable for use in electric vehicles and plug-in hybrids are tightly patented and have not been licensed for use by manufacturers, thereby slowing the development of new models.

Advantages:

• Success of the Ni-MH has been driven by its high energy density and the use of environmentally friendly metals. The modern Ni-MH offers up to 40 percent higher energy density compared to nickel cadmium (NiCd) battery.
• Potential for yet higher energy densities 
• Less prone to memory than the NiCd 
• Periodic exercise cycles are required less often
• Simple storage and transportation; transportation conditions are not subject to regulatory control. 
• Environmentally friendly; contains only mild toxins
• Profitable for recycling

Limitations:

• High self-discharge: Ni-MH has about 50 % higher self-discharge compared to the NiCd. New chemical additives improve the self-discharge but at the expense of lower energy density.
• Performance degrades if stored at elevated temperatures: Ni-MH should be stored in a cool place and at a state-of-charge of about 40 percent.
• High maintenance: Battery requires regular full discharge to prevent crystalline formation.
• About 20% more expensive than NiCd: Ni-MH batteries designed for high current draw are more expensive than the regular version.

Performance: The nickel-metal hydride battery makes use of hydrogen for the positive electrode. This hydrogen is stored in alloy (i.e., metal hydride). Nickel–metal hydride batteries have a similar energy and power performance as nickel–zinc batteries. However, the cycle life performances are much higher (> 1000 cycles). In the last two decades, nickel–metal hydride batteries have been used as a high power source in several commercial hybrid vehicles.

Price range: In India, the approximate price is Rs 725 / piece (6V, 2200 mAh) [as of March 22, 2022].

Commercialization: The first consumer-grade Ni-MH cells became commercially available in 1989. In 1994, GM and Ovonic Battery Company formed a manufacturing joint venture, GM Ovonic, to commercialize Ni-MH batteries for EVs. GM has subsequently incorporated GM Ovonic’s Ni-MH batteries in over 400 of their 1999 model EV-1 cars and S-10 pickups, in which driving range was doubled for both vehicles.

In 1997, DaimlerChrysler announced its decision to equip its Electric Power Interurban Commuter with Ni-MH batteries made by Saft, which marks the first use of Ni-MH batteries in a minivan. The Saft Ni-MH battery helped reduce the weight of the minivan by 150 pounds and increased its driving range up to 150 km. Both manufacturers are now working on the production of second-generation Ni-MH batteries.