In the 1970s research scientists started to look at how the NiMH Battery system worked as a means of discovering how to store hydrogen for the nickel hydrogen battery.
Things have moved on a lot since then and Nickel Hydrogen batteries today, are mainly used for satellite applications. Unfortunately, they tend to be quite size-able, require high-pressure steel canisters and can cost many thousands of pounds. In the embryonic days of NiMH batteries, the metal hydride alloys used were unstable in the cell environment and the performance characteristics that were needed could not be achieved.
As a result, the development of the NiMH battery pack slowed and in some instances ground to a halt. Subsequently, in the 1980s, new hydride alloys were developed that were stable enough for use in a battery cell.
Since this time, NiMH battery pack engineering has steadily improved, mainly in terms of energy density and reliability.
The recent success of NiMH Batteries has been driven by its high energy density and the use of environmentally friendly metals. The modern NiMH Battery Pack offers up to 40 percent higher energy density compared to its rival, the Ni-Cd Battery Cell. Longer term there is potential for yet higher capacities, but not without some negative side effects.
Both NiMH and Ni-Cd are affected by high self-discharge. The Ni-Cd 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 cell is about one-and-a-half to two times greater compared to Ni-Cd. 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.
Over time, NiMH Batteries have been replacing the Ni-Cd Cells in markets such as wireless communications and mobile computing. In many parts of the world, the buyer is encouraged to use NiMH rather than Ni-Cd 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 Ni-Cd battery. It is widely accepted that NiMH is an interim step to lithium battery technology.
Initially more expensive than the Ni-Cd, 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 Ni-Cd, there will be a tendency for the price to increase. PMBL can manufacture your NiMH batteries with cells from Varta, GP, Saft, Sanyo, Emmerich and many more.
Advantages and Limitations of NiMH Batteries Advantages
- 30 - 40 % higher capacity over a standard Ni-Cd. The NiMH has potential for yet higher energy densities.
- Less prone to memory than the Ni-Cd. 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.
- 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 batterys 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 Ni-Cd. 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 Ni-Cd. 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 %
- High maintenance - battery requires regular full discharge to prevent crystalline formation.
- About 20 % more expensive than Ni-Cd - NiMH batteries designed for high current draw are more expensive than the regular version.
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