Secondary Batteries: Nickel-cadmium and nickel metal hydride

Nickel-based batteries account for a one of the larger battery categories used in both the consumer and commercial sectors. Nickel-cadmium (Ni-Cd) and nickel metal hydride (Ni-MH) batteries are secondary batteries, which in simple terms, means that they are rechargeable, as opposed to single-use, primary batteries (e.g., alkaline batteries, silver-oxide watch batteries, and others).

Since the 1940s, portable devices were often powered by nickel-cadmium batteries, but this chemistry was supplanted by the rise of the nickel metal hydride battery in the nineties, coinciding with environmental concerns and the ability to better meet various applications. Despite their differences, the two systems share similarities. Ni-MH batteries, in fact, inherited many of the characteristics of Ni-Cd batteries that they came to replace.

Nickel-cadmium batteries (Ni-Cd)

As one of the older battery technologies in use, Nickel-cadmium batteries were invented in 1899 by Waldemar Jungner during a time when the only other rechargeable power in active use was the lead-acid battery.  Despite the cost of materials and slow development, the Ni-Cd offered several advantages over lead-acid batteries.  In 1932, advancements were made to deposit the active materials inside a porous nickel-plated electrode.  By 1947, Ni-Cd was capable of absorbing the gases generated during charge, propelling the technology to the sealed Ni-Cd battery.

Nickel-cadmium grew to be the preferred battery choice for biomedical and emergency medical equipment, professional video cameras, two-way radios, power tools, and a variety of other devices.  In the late 1980s, Ni-Cd batteries made an impact with models that packed more active material in the cell which enabled capacities of up to 60 percent higher than the standard Ni-Cd; however, these ultra-high-capacity Ni-Cd batteries suffered higher internal resistance and reduced cycle count compared to the standard versions. 

Ni-Cd batteries continued to be a staple battery for portable devices until the 1990s.  The ruggedness of the Ni-Cd made it among the more robust and forgiving battery of its time. By modern standards, they continue to be considered durable batteries that work well in rigorous applications where long life, high discharge rate, and economical pricing are the priorities. But because they are composed of cadmium and other toxic metals, they have presented disposal issues. Nickel cadmium batteries are not a model for environmental friendliness and are now largely designated to commercial applications. The airline industry, for example, continues to turn to Ni-Cd. 

Both Ni-Cd and the newer Ni-MH, suffer from a ‘memory effect’. In simple terms, memory effect is a condition in which a battery seems to remember the previous energy delivered and fails to deliver beyond it.  This requires a periodic full discharge cycle to offset the battery’s loss of capacity over time due to memory effect. 

 

Ni-Cd battery advantages

Ni-Cd battery disadvantages/limitations

High number of charge and discharge cycles with proper maintenance. Most economical battery type in terms of operational cost per cycle. Wide selection of sizes and performance options. Shortest charge time, even after extended storage or at low temperatures. Delivers highest load current. Easy to charge and handles overcharges and rapid charging with minimal stress. Rugged and able to withstand abuse. Long shelf life in nearly any state of charge.

Holds a low energy density relative to newer battery technologies. Its low voltage requires numerous cells to achieve same voltage available with other batteries. The most susceptible to memory effect. Moderately high self-discharge rate in storage. The toxicity of its cadmium chemistry renders it environmentally unfriendly and ill-suited for landfills.

Nickel metal hydride batteries (Ni-MH)

Initial research for nickel metal hydride began in the late 1960s, however, development was hindered by difficulties due to instabilities with metal-hydride. Subsequently, research shifted to the development of the nickel-hydrogen (Ni-H) battery.  Ni-H stores its hydrogen in a steel canister at a pressure of 8,270kPa (1,200psi) and maintains a nominal cell voltage of 1.25V.  Its long service life (even at full discharge cycles), good calendar life due to low corrosion, minimal self-discharge, and a remarkable temperature performance of –28°C to 54°C, made Ni-H suitable for satellite installation. Unfortunately, its low specific energy (40–75Wh/kg) and high cost hindered advancements toward practical terrestrial use.

In the 1980’s newly discovered hydride alloys solved the stability issues previously encountered and facilitated the production of the nickel metal hydride battery as the quasi-replacement of the Ni-CD batteries. While they retained plenty of their predecessor’s favorable traits, Ni-MH batteries presented a notable drawback—a high self-discharge rate of 20 percent within the first 24 hours followed by a high 10 percent per month.  They also proved to be more delicate and trickier to charge than Ni-Cd batteries.  To address these issues, hydride materials were modified to lower self-discharge and alloy corrosion, but this subsequently decreased the specific energy.  The robustness and longevity achieved by this modification, however, made them suitable for powering electric powertrain.

Today, nickel metal hydride batteries are able to provide 40 percent higher specific energy than their Ni-Cd counterparts. Well recognized in the consumer sector, the Ni-MH battery has also become one of the most readily usable, rechargeable battery solutions due to its durability, low cost, and assortment of sizes available including AA, AAA, C, and D. It also serves as a viable alternative for both primary disposable and the obsolete reusable alkaline batteries. 

The Ni-MH battery family is widely viewed as a step toward lithium battery technology. The need for higher energy densities and a less toxic composition supported the drive toward newer and more advanced battery chemistries. The nickel metal hydride battery provides defined improvements in these areas. Moreover, with technical advancements, the modern Ni-MH battery suffers minimal to none of the memory effects that plague its nickel-cadmium counterpart. Encouragement of continued development and its use has largely been fueled by environmental concerns about careless disposal; however, it continues to suffer some of the shortcomings native to the nickel-based chemistry.

With the availability of newer rechargeable lithium technologies and advancements with single-use primary batteries, its limited cycle life and loading characteristics have made it less remarkable. Rechargeable batteries have higher self-discharge rate during storage, which Ni-MH batteries are known for, unlike primary alkaline and lithium batteries which can keep their charge for 10 years.  When it comes to devices designed for emergencies or instant use, such as flashlights, the constant need to recharge prior to use is a substantial inconvenience, if not a detriment.  Manufacturers including Energizer, Duracell, Panasonic, and others continue to successfully develop the battery to improve performance and reduce self-discharge. For example, Panasonic’s Eneloop Ni-MH batteries, which reportedly perform well in cold temperatures, has made substantial advancements with changes in chemical composition and a modified separator to reduce their self-discharge. You can store the charged Eneloop battery six times longer than a regular Ni-MH before a recharge is necessary.

With the differences between Ni-Cd and Ni-MH boiling down to capacity, memory effect, and environmental friendliness, it is up to the user to weigh the needs and drawbacks.

 

Ni-MH battery advantages (over standard Ni-Cd batteries)

Ni-MH battery disadvantages/limitations (over standard Ni-Cd batteries)

Improved energy density with a higher capacity of roughly 30 to 40 percent. Current models are substantially less vulnerable to memory issues. Require lower maintenance. Transportation conditions are less regulated which simplifies transportation and storage. Excellent safety performance. Environmentally friendly with fewer toxins.

More expensive, although the pricing gap is decreasing. Requires attention to prevent crystal formation that negatively impacts performance. Suffers performance degradation when stored in warmer temperatures. As much as 50 percent higher self-discharge. Technology improvements have been at the cost of lowering energy density. Longer charge time that requires careful control of trickle charging. Repeated deep discharges or cycling at high currents deteriorate the battery after a few hundred cycles, limiting both its service life and cycle duration

Other notable nickel chemistry batteries

Nickel iron batteries (Ni-Fe). Swedish inventor, Waldemar Junger, who invented nickel-cadmium in 1899, also tried to substitute cadmium with iron. Development was hampered by poor charge efficiency and hydrogen formation. Thomas Edison continued its development in 1901. Edison claimed that immersion of nickel-iron in an alkaline electrolyte was superior to batteries that used lead plates immersed in sulfuric acid, making it the choice for electric vehicles. However, the rise of gasoline-powered car, which used the lead-acid battery for the starter, lighting, and ignition (SLI), rendered his nickel-iron battery of no practical use.

The nickel-iron battery (Ni-Fe) can last over 50 years, and at least 20 years in standby applications as it is resilient to overcharge and over-discharge. Lead-acid batteries in cycling mode last less than 12 years. Because Ni-Fe batteries are also resistant to vibrations and high temperatures, they were ideal for mining, railroad signaling, forklifts, and flying bombs during World War II.  However, Ni-Fe has a low specific energy of 50Wh/kg, performs poorly at cold temperatures, and maintains a high monthly self-discharge rate of 20-40 percent. It also costs four times as much as lead-acid batteries to manufacture and is in the vicinity of Li-ion batteries in purchase price. Ni-Fe is currently being improved using pocket plate technology to lower the self-discharge as a potential lead-acid alternative for off-grid power systems.

Nickel-zinc batteries (Ni-Zn). The recyclable nickel-zinc battery was patented to Thomas Edison in 1901. Its use was implemented in rail cars from 1932 to 1948.

Nickel-zinc batteries provide 1.65V/cell voltage in contrast to the lower 1.2V/cell delivered by Ni-Cd and Ni-MH batteries.  In common with Ni-Cd batteries, it uses use an alkaline electrolyte and a nickel electrode.  Nickel-zinc batteries have double the specific energy of Ni-Fe batteries and can be cycled 200–300 times.  Ni-Zn batteries cannot accept a maintenance, or trickling, however, and should be charged at a constant current of 1.9V/cell.  The battery does not consist of heavy toxic materials which facilitates ease of recycling.  It also boasts of low cost, high power output, and a good temperature operating range.

Despite its benefits, the nickel-zinc battery suffered drawbacks which included a high self-discharge rate and short life cycle due to dendrite growth which contributed to frequent electrical shorts. Improvements were made in the battery’s electrolyte making it appealing for commercial uses.

 

Learn about another large secondary battery type, the rechargeable lithium battery, in our upcoming article below:

Secondary battery options: lithium-ion rechargeable batteries