The nickel-metal hydride battery is less durable than the nickel-cadmium battery, as cycling under heavy load and storage at high temperature reduces its service life. Nickel-metal hydride also 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.

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Advantages

• Fast and simple charge, even after prolonged storage
• High number of charge/discharge cycles – and 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 available
• 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
• Relatively high self-discharge – needs recharging after storage

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Nickel-cadmium – mature but has moderate energy density. Nickel-cadmium is used where long life, high discharge rate and extended temperature range is important. Main applications are two-way radios, biomedical equipment and power tools. Nickel-cadmium contains toxic metals.

Nickel-metal-hydride – has a higher energy density compared to nickel-cadmium at the expense of reduced cycle life. There are no toxic metals. Applications include mobile phones and laptop computers. NiMH is viewed as a steppingstone to lithium-based systems.

Lead-acid – most economical for larger power applications where weight is of little concern. Lead-acid is the preferred choice for hospital equipment, wheelchairs, emergency lighting and UPS systems. Lead acid is inexpensive and rugged. It serves a unique niche that would be hard to replace with other systems.

Lithium-ion – fastest growing battery system; offers high-energy density and low weight. Protection circuits are needed to limit voltage and current for safety reasons. Applications include notebook computers and cell phones. High current versions are available for power tools and medical devices.

It’s important to note that the nickel-cadmium battery is the first rechargeable battery in small format and forms a standard against which other chemistries are commonly compared. The trend is toward lithium-based systems.

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Most defibrillators are powered by nickel-cadmium batteries. Nickel-metal-hydride is also used but there is the concern of short service life. Sealed lead-acid batteries are often used to power defibrillators intended for standby mode. Although bulky and heavy, the lead-acid battery has a low self-discharge and can be kept in prolonged ready mode without the need to recharge. Lead-acid batteries perform well on high current spurts. During the rest periods the battery disperses the depleted acid concentrations back into the electrode plate. Lead-acid batteries would not be suitable for a sustained high load.

The medical industry is moving toward lithium-ion. The short but high current spurts needed for defibrillators are still a challenge. Paralleling the cells and adding current-limiting circuits that allow short spikes of high current will help overcome this hurdle.

Sometimes a battery can be the difference between life and death… so make sure it is charged.

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With charge only (charge-and-use), the annual percentage of battery failure was 45%. With exercise, the failure rate was reduced to 15%. By far the best results were achieved with recondition, where failure rate dropped to 5%.

The GTE report concluded that a battery analyzer featuring exercise and recondition functions costing $2,500 would return its investment in less than one month on battery savings alone.

So, to wrap up our segment on battery memory, exercise and reconditioning, here are a few simple guidelines:

• Do not leave a nickel-based battery in a charger for more than a few days, even on trickle charge.
• Exercise nickel-cadmium batteries every 1 to 2 months and nickel-metal-hydride batteries every 3 months. Running the battery down in the equipment may do this also.
• Do not discharge the battery before each recharge. This puts undue stress on the battery.
• Avoid getting the battery too hot during charge. The temperature should only rise for a short moment at full charge, then cool off.

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Nickel-cadmium batteries in regular use and on standby mode (sitting in a charger for operational readiness) should be exercised once per month. Between these monthly exercise cycles, no further service is needed. No scientific research is available on the optimal exercise requirements of nickel-metal-hydride batteries but based on the reduced crystalline buildup, applying a full discharge once every three months appears appropriate. Because of the shorter cycle life of the nickel-metal-hydride batteries compared to nickel-cadmium, over-exercising is not recommended.

So, yes, exercise is ALWAYS healthy – even for your batteries!

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The problem with nickel-cadmium batteries is not so much the cyclic memory but the effects of crystalline formation. The active cadmium material is present in finely divided crystals. In a good battery, these crystals remain small, obtaining maximum surface area. With memory, however, the crystals grow and conceal the active material from the electrolyte. In advanced stages, the sharp edges of the crystals may even penetrate the separator, causing high self-discharge or electrical short.

When introduced in the early 1990s, nickel-metal-hydride was promoted as being memory-free. Today, we know that this chemistry is also affected but to a lesser degree than nickel-cadmium. The nickel plate, a metal that is shared by both chemistries, is partly to blame. While nickel-metal-hydride has only the nickel plate to worry about, nickel-cadmium also includes the memory-prone cadmium plate. This is a non-scientific explanation why nickel-cadmium is affected more than nickel-metal-hydride.

So, as with our own memories, sometimes it is best not to remember – but to forget and move on!

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In North America, some recycling plants bill on weight. The rates vary according to chemistry. Systems that yield high metal retrieval rates from batteries are priced lower than those which produce less valuable metals.

Nickel-metal-hydride yields the best return. It produces enough nickel to pay for the process. The highest recycling fees apply to nickel-cadmium and lithium ion batteries because the demand for cadmium is low and lithium-ion contains little retrievable metal.

Not all countries base the cost of recycling on the battery chemistry; some put it on tonnage alone. The flat cost to recycle batteries is about $1,000 to $2,000 per ton. Europe hopes to achieve a cost per ton of $300. Ideally, this would include transportation; however, moving the goods is expected to double the overall cost. For this reason, Europe sets up several smaller processing locations in strategic geographic locations.

The bottom line is that significant subsidies are still required from manufacturers, agencies and governments to support battery recycling programs. This funding is in the form of a tax added to each manufactured cell.

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The recycling process starts by removing the combustible material, such as plastics and insulation, with a gas fired thermal oxidizer. Gases from the thermal oxidizer are sent to the plant’s scrubber where they are neutralized to remove pollutants. The process leaves the clean, naked battery cells, which contain valuable metal content.

The battery cells are then chopped into small pieces, which are heated until the metal liquefies. Non-metallic substances are burned off; leaving a black slag on top that is removed with a slag arm. The different alloys settle according to their weights and are skimmed off .Cadmium is relatively light and vaporizes at high temperatures. In a process that appears like a pan boiling over, a fan blows the cadmium vapor into a large tube, which is cooled with a water mist. This causes the vapors to condense and produces cadmium that is 99.95 percent pure.

Some recyclers do not separate the metals on site but pour the liquid metals directly into what the industry refers to as ‘pigs’ (65 pounds) or ‘hogs’ (2000 pounds). The pigs and hogs are then shipped to metal recovery plants. Here, the material is used to produce nickel, chromium and iron re-melt alloy for the manufacturing of stainless steel and other high-end products.Current battery recycling methods requires a high amount of energy. It takes six to ten times the amount of energy to reclaim metals from recycled batteries than it would through other means.

Stay tuned next week for information on who pays for this recycling…

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Careless disposal of nickel-cadmium is hazardous to the environment. If used in landfills, the cadmium will eventually dissolve itself and the toxic substance can seep into the water supply, causing serious health problems. Our oceans are already beginning to show traces of cadmium (along with aspirin, penicillin and antidepressants) but the source of the contamination is unknown.

Although nickel-metal-hydride is considered environmentally friendly, this chemistry is also being recycled. If no disposal service is available in an area, individual nickel-metal-hydride batteries can be discarded with other household wastes.

Lithium (metal) batteries contain no toxic metals; however, there is the possibility of fire if the metallic lithium is exposed to moisture while the cells are corroding. Most lithium batteries are non-rechargeable and are used in cameras, hearing aids and defense applications. For proper disposal, the batteries must first be fully discharged to consume the metallic lithium content. Lithium-ion batteries used for cell phones and laptops do not contain metallic lithium and the disposal problem does not exist.

Stay tuned next week for more on recycling…

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