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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.
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Advantages
and Limitations of NiMH Batteries
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Advantages
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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.
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Limitations
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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.
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Figure 2-3:
Advantages and limitations of NiMH batteries
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