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In this section we evaluate the strengths and limitations of various battery chemistries, beginning with the nickel. Each battery system offers unique advantages but none provides a fully satisfactory solution. With the increased selection of battery chemistries available today, better choices can be made to address specific battery needs. A careful evaluation of each battery's attribute is important. Because of similarities, both nickel-cadmium and nickel-metal hydride are covered in this paper.

The nickel-cadmium battery

Swedish Waldmar Jungner invented the nickel-cadmium battery in 1899. At that time, the materials were expensive compared to other battery types available 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.

Rather than venting, the internal gases generated during charge were recombined. 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 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. There is shift towards batteries with higher energy densities and less toxic metals but alternative chemistries cannot always match the superior durability and low cost of nickel-cadmium.

Here is a summary of the advantages and limitations of nickel-cadmium batteries.

Advantages

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 - 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 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.
Limitations

Relatively low energy density.

Memory effect - nickel-cadmium must periodically be exercised (discharge/charge) to prevent memory.

Environmentally unfriendly - nickel-cadmium contains toxic metals. Some countries restrict its use.

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. nickel hydrogen batteries are bulky, require high-pressure steel canisters and cost thousands of dollars per cell.

In the early experimental days of nickel-metal hydride, 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 then, nickel-metal hydride has steadily improved.

The success of nickel-metal hydride has been driven by high energy density and the use of environmentally friendly metals. The modern nickel-metal hydride offers up to 40% higher energy density compared to the standard nickel-cadmium. There is potential for yet higher capacities, but not without some negative side effects.

Nickel-metal hydride is less durable than 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 higher than that of nickel-cadmium.

Nickel-metal hydride has been replacing nickel-cadmium in markets such as wireless communications and mobile computing. Experts agree that nickel-metal hydride has greatly improved over the years, but limitations remain. Most shortcomings are native to the nickel-based technology and are shared with nickel-cadmium. It is widely accepted that nickel-metal hydride is an interim step to lithium-based battery technology.

Here is a summary of the advantages and limitations of nickel-metal hydride batteries.

Advantages


30-40% higher capacity than 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.
Limitations

Limited service life - the performance starts to deteriorate after 200-300 cycles if repeatedly deeply cycled.

Relatively short storage of three years. Cool temperature and a partial charge slows aging.

Limited discharge current - although nickel-metal-hydride is capable of delivering high discharge currents, heavy load reduces the battery's cycle life.

More complex charge algorithm needed - nickel-metal-hydride generates more heat during charge and requires slightly longer charge times than nickel-cadmium. Trickle charge settings are critical because the battery cannot absorb overcharge.

High self-discharge - typically 50% higher than nickel-cadmium.

Performance degrades if stored at elevated temperatures - nickel-metal-hydride should be stored in a cool place at 40% state-of-charge.

High maintenance - nickel-metal hydride requires regular full discharge to prevent crystalline formation. nickel-cadmium should be exercised once a month, nickel-metal-hydride once in every 3 months.

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