Batteries in a Portable World 2^nd Ed.
       A Handbook on Rechargeable Batteries for Non-Engineers

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2.3 The Nickel-Metal Hydride (NiMH) Battery

Research of the NiMH system started in the 1970s as a means of discovering how to store hydrogen for the nickel hydrogen battery. Today, nickel hydrogen batteries are mainly used for satellite applications. They are bulky, contain high-pressure steel canisters and cost thousands of dollars each.

In the early experimental days of the NiMH battery, the metal hydride alloys were unstable in the cell environment and the desired performance characteristics could not be achieved. As a result, the development of the NiMH slowed down. New hydride alloys were developed in the 1980s that were stable enough for use in a cell. Since the late 1980s, NiMH has steadily improved, mainly in terms of energy density.

The success of the NiMH has been driven by its high energy density and the use of environmentally friendly metals. The modern NiMH offers up to 40 percent higher energy density compared to NiCd. There is potential for yet higher capacities, but not without some negative side effects.

Both NiMH and NiCd are affected by high self-discharge. The NiCd loses about 10 percent of its capacity within the first 24 hours, after which the self-discharge settles to about 10 percent per month. The self-discharge of the NiMH is about one-and-a-half to two times greater compared to NiCd. Selection of hydride materials that improve hydrogen bonding and reduce corrosion of the alloy constituents reduces the rate of self-discharge, but at the cost of lower energy density.

The NiMH has been replacing the NiCd in markets such as wireless communications and mobile computing. In many parts of the world, the buyer is encouraged to use NiMH rather than NiCd batteries. This is due to environmental concerns about careless disposal of the spent battery.

The question is often asked, “Has NiMH improved over the last few years?” Experts agree that considerable improvements have been achieved, but the limitations remain. Most of the shortcomings are native to the nickel-based technology and are shared with the NiCd battery. It is widely accepted that NiMH is an interim step to lithium battery technology.

Initially more expensive than the NiCd, the price of the NiMH has dropped and is now almost at par value. This was made possible with high volume production. With a lower demand for NiCd, there will be a tendency for the price to increase.

Advantages and Limitations of NiMH Batteries
Advantages	30 – 40 percent higher capacity over a standard NiCd. The NiMH has 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	Limited service life — if repeatedly deep cycled, especially at high load currents, the performance starts to deteriorate after 200 to 300 cycles. Shallow rather than deep discharge cycles are preferred. Limited discharge current — although a NiMH battery is capable of delivering high discharge currents, repeated discharges with high load currents reduces the battery’s cycle life. Best results are achieved with load currents of 0.2C to 0.5C (one-fifth to one-half of the rated capacity). More complex charge algorithm needed — the NiMH generates more heat during charge and requires a longer charge time than the NiCd. The trickle charge is critical and must be controlled carefully. High self-discharge — the NiMH has about 50 percent 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 — the NiMH 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 percent more expensive than NiCd — NiMH batteries designed for high current draw are more expensive than the regular version.

Figure 2-3:       Advantages and limitations of NiMH batteries

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