Q: What are the chraracteristics of Nickel-based batteries, its dominance and the future?
In the next monthly columns we evaluate the strengths
and limitations of nickel, lithium and lead-based battery
systems. Each of these batteries offers unique advantages but none provides a fully satisfactory solution. With the increased selection in battery chemistries available, however, better choices can be made to address specific battery needs.
Nickel-cadmium and Nickel-metal hydride are similar in many ways, the second being the newer of the two. For the purpose of evaluation, we begin by addressing both systems in this column.
The Nickel-cadmium battery
Alkaline nickel battery technology originated in 1899, when Waldmar Jungner invented the Nickel-cadmium battery. The materials were expensive compared to other battery types available at the time and its use was limited to special applications. In 1932, the active materials were deposited inside a porous nickel-plated electrode and in 1947, research began on a sealed Nickel-cadmium battery, which recombined the internal gases generated during charge rather than venting them. These advances led to the modern sealed Nickel-cadmium battery, which is in use today.
Nickel-cadmium prefers fast charge to slow charge and pulse charge to DC charge. It is a strong and silent worker; hard labor with repeated full discharges poses little problem. In fact, Nickel-cadmium is the only battery type that performs well under rigorous working conditions. All other chemistries prefer a shallow discharge and moderate load currents.
Nickel-cadmium does not like to be pampered by sitting in chargers for days and being used only occasionally for brief periods. A periodic full discharge is so important that, if omitted, large crystals will form on the cell plates (also referred to as memory) and the Nickel-cadmium will gradually lose its performance.
Among rechargeable batteries, Nickel-cadmium remains a popular choice for two-way radios, emergency medical equipment and power tools. Batteries with higher energy densities and less toxic metals are causing a diversion from Nickel-cadmium to newer technologies.
Here is a summary of the Advantages and Limitations of Nickel-cadmiumbatteries.
· Fast and simple charge, even after prolonged storage.
· High number of charge/discharge cycles if properly maintained. Nickel-cadmium provides over 1000 charge/discharge cycles.
· Good load performance the Nickel-cadmium allows recharging at low temperatures.
· Long shelf life five-year storage is possible. Some priming prior to use will be required.
· Simple storage and transportation most airfreight companies accept the Nickel-cadmium without special conditions.
· Good low temperature performance.
· Forgiving if abused Nickel-cadmium is one of the most rugged rechargeable batteries.
· Economically priced Nickel-cadmium is lowest in terms of cost per cycle.
· Available in a wide range of sizes and performance options most Nickel-cadmium cells are cylindrical.
· Relatively low energy density.
· Memory effect Nickel-cadmium must periodically be exercised to prevent memory.
· Environmentally unfriendly Nickel-cadmium contains toxic metals. Some countries restrict its use.
· Has relatively high self-discharge needs recharging after storage.
The Nickel-metal-hydride battery
Research on the Nickel-metal-hydride system started in the 1970s as a means of storing hydrogen for the nickel hydrogen battery. Today, nickel hydrogen is used mainly for satellite applications. The system is bulky, requires high-pressure steel canisters and costs thousands of dollars per cell.
In the early experimental days of the Nickel-metal-hydride 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 Nickel-metal-hydride slowed down. New hydride alloys were developed in the 1980s that were stable enough for use in a cell. Since the late 1980s, Nickel-metal-hydride has steadily improved.
The success of Nickel-metal-hydride has been driven by its high energy density and the use of environmentally friendly metals. The modern Nickel-metal-hydride offers up to 40 percent higher energy density compared to Nickel-cadmium. There is potential for yet higher capacities, but not without some negative side effects.
Nickel-metal-hydride is less durable than the Nickel-cadmium. Cycling under heavy load and storage at high temperature reduces the service life. Nickel-metal-hydride suffers from high self-discharge, which is considerably greater than that of Nickel-cadmium.
Nickel-metal-hydride has been replacing Nickel-cadmium in markets such as wireless communications and mobile computing. In many parts of the world, the buyer is encouraged to use Nickel-metal-hydride rather than Nickel-cadmium batteries. This is due to environmental concerns about careless disposal of the spent battery.
Experts agree that Nickel-metal-hydride has greatly improved over the years, but limitations remain. Most of the shortcomings are native to the nickel-based technology and are shared with the Nickel-cadmium battery. It is widely accepted that Nickel-metal-hydride is an interim step to lithium battery technology.
Here is a summary of the Advantages and Limitations of Nickel-metal-hydride batteries.
· 30 40 percent higher capacity over standard Nickel-cadmium. Nickel-metal-hydride has potential for yet higher energy densities.
· Less prone to memory than Nickel-cadmium fewer exercise cycles are required.
· Simple storage and transportation transport is not subject to regulatory control.
· Environmentally friendly contains only mild toxins; profitable for recycling.
· Limited service life if repeatedly deep cycled, the performance starts to deteriorate after 200 to 300 cycles.
· Limited discharge current although Nickel-metal-hydride is capable of delivering high discharge currents, heavy load 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 Nickel-metal-hydride generates more heat during charge and requires a longer charge time than the Nickel-cadmium. Trickle charge settings are critical because the battery cannot absorb overcharge.
· High self-discharge typically 50 percent higher than Nickel-cadmium. New chemical additives have improved self-discharge but at the expense of lower energy density.
· Performance degrades if stored at elevated temperatures Nickel-metal-hydride 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. Nickel-cadmium should be exercised once a month, Nickel-metal-hydride once in every 3 months.
Isidor Buchmann, founder and CEO,
Cadex Electronics, Inc.