Learn about the differences in nickel-cadmium and nickel-metal-hydride.
Nickel-cadmium (NiCd) was the first battery in this family, and 50 years of usage generated extensive data. Many of the characteristics of the NiCd are shared with nickel-metal-hydride (NiMH), as these two systems are close cousins. NiMH is the younger partner that was developed in part to solve the toxicity problem of the otherwise solid and robust NiCd. This limits NiCd to specialty applications.
The nickel-cadmium battery, invented by Waldmar Jungner in 1899, offered several advantages over lead acid, but the materials were expensive and the early use was restricted. Developments lagged until 1932 when attempts were made to deposit the active materials inside a porous nickel-plated electrode. Further improvements occurred in 1947 by trying to absorb the gases generated during charge. This led to the modern sealed NiCd battery in use today.
For many years, NiCd was the preferred battery choice for two-way radios, emergency medical equipment, professional video cameras and power tools. In the late 1980s, the ultra-high-capacity NiCd rocked the world with capacities that were up to 60 percent higher than the standard NiCd. This was done by packing more active material into the cell, but the gain was offset with higher internal resistance and a reduced cycle count.
The standard NiCd remains one of the most rugged and forgiving batteries but it needs proper care to attain longevity. NiCd, and in part also NiMH, have memory effect that causes a loss of capacity if not given a periodic full discharge cycle. The battery appears to remember the previous energy delivered and once a routine has been established it does not want to give more. (See BU-807: How to Restore Nickel-based Batteries) Table 1 lists the advantages and limitations of the standard NiCd.
Fast and simple charging even after prolonged storage
High number of charge/discharge cycles; provides over 1,000 charge/discharge cycles with proper maintenance
Good load performance; rugged and forgiving if abused
Long shelf life; can be stored in a discharged state
Simple storage and transportation; not subject to regulatory control
Good low-temperature performance
Economically priced; NiCd is the lowest in terms of cost per cycle
Available in a wide range of sizes and performance options
Relatively low specific energy compared with newer systems
Memory effect; needs periodic full discharges
Cadmium is a toxic metal. Cannot be disposed of in landfills
High self-discharge; needs recharging after storage
Table 1: Advantages and limitations of NiCd batteries
Research of nickel-metal-hydride started in 1967; however, instabilities with the metal-hydride led scientists to develop the nickel-hydrogen battery (NiH) instead. NiH was mainly used in satellites and is being replaced by long-life Li-ion. See BU-211: Alternate Battery Systems).
New hydride alloys discovered in the 1980s offered better stability and the development of NiMH advanced in earnest. Today, NiMH provides 40 percent higher specific energy than a standard NiCd, but the decisive advantage is the absence of toxic metals. Much of the research on NiMH has been reduced or discontinued in favor of lithium-ion.
Nickel-metal-hydride is not without drawbacks. They are more delicate and fuzzier to charge than NiCd. They also have high self-discharge and lose about 20 percent of its capacity within the first 24 hours, and 10 percent per month thereafter. Modifying the hydride materials lowers the self-discharge and reduces corrosion of the alloy, but this decreases the specific energy. Batteries for the electric powertrain make use of this modification to achieve the needed robustness and life span.
NiMH has become one of the most readily available and low-cost rechargeable batteries for portable devices. Battery manufacturers, such as Panasonic, Sanyo, Energizer, Duracell and Rayovac, have recognized the need for a durable and low-cost rechargeable battery and offer NiMH in AA and AAA sizes. The battery manufacturers hope to persuade buyers to switch to rechargeable batteries and reduce the environmental impact of throwaway primary cells.
The NiMH battery for the consumer market is an alternative for the failed reusable alkaline that appeared in the 1990s. Limited cycle life and poor loading characteristics hindered its success.
Table 2 compares the specific energy, voltage, self-discharge and runtime of over-the-counter batteries. Available in AA, AAA and other sizes, these cells can be used in portable devices designed for these norms. Even though the cell voltages may vary, the end-of-discharge voltages are common, which is typically 1V/cell. Portable devices have some flexibility in terms of voltage range. It is important not to mix and match cells and to always use the same type of batteries in the holder. Safety concerns and voltage incompatibility prevent the sales of lithium-ion batteries in AA and AAA formats.
1 year storage
on digital camera
|NiMH||2,700mAh, rechargeable||1.2V||50%||600 shots|
|Eneloop*||2,400mAh, rechargeable||1.2V||85%||500 shots|
|Reusable alkaline||2,000mAh; lower on subsequent recharge||1.4V||95%||100 shots|
Table 2: Comparison of alkaline, reusable alkaline, Eneloop and NiMH
* Eneloop is a Sanyo trademark, based on NiMH.
High self-discharge is of ongoing concern to consumers using rechargeable batteries, and NiMH behaves like a leaky basketball or bicycle tire. A flashlight or portable entertainment device with a NiMH battery gets “flat” when put away for only a few weeks. Having to recharge the device before each use does not sit well.
The Eneloop NiMH by Panasonic and Sanyo has reduced the self-discharge by a factor of six. This means you can store the charged battery six times longer than a regular NiMH before a recharge becomes necessary. The drawback of the Eneloop to regular NiMH is a slightly lower specific energy compared.
Table 3 summarizes the advantages and limitations of industrial-grade NiMH. The table does not include the Eneloop and equivalent consumer brands.
30–40 percent higher capacity than a standard NiCd
Less prone to memory than NiCd
Simple storage and transportation; not subject to regulatory control
Environmentally friendly; contains only mild toxins
Nickel content makes recycling profitable
Limited service life; deep discharge reduces service life
Requires complex charge algorithm
Does not absorb overcharge well; trickle charge must be kept low
Generates heat during fast-charge and high-load discharge
Table 3: Advantages and limitations of NiMH batteries
After inventing nickel-cadmium in 1899, Sweden’s Waldemar Jungner tried to substitute iron for cadmium to save money; however, poor charge efficiency and gassing (hydrogen formation) prompted him to abandon the development without securing a patent.
In 1901, Thomas Edison continued the development of the nickel-iron battery as a substitute to lead acid for electric vehicles. He claimed nickel-iron was “far superior to batteries using lead plates and acid” and counted on the emerging electric vehicle market. He lost out when gasoline-powered cars took over and was deeply disappointed when the auto industry did not adopt nickel-iron as the starter, lighting and ignition battery (SLI) for cars. (See BU-1203: Electric Vehicle)
The nickel-iron battery (NiFe) uses an oxide-hydroxide cathode and an iron anode with potassium hydroxide electrolyte and produces a nominal cell voltage of 1.2V. NiFe is resilient to overcharge and over-discharge and can last for more than 20 years in standby applications. Resistance to vibrations and high temperatures made NiFe the preferred battery for mining in Europe, and during World War II the battery powered the German V-1 flying bomb and the V-2 rockets. Other uses are railroad signaling, forklifts, and power for stationary applications.
NiFe has a low specific energy of about 50Wh/kg and is similar to lead acid, has poor low-temperature performance and exhibits high self-discharge of 20 to 40 percent a month. This, with high manufacturing cost, prompted the industry to stay faithful to lead acid.
Improvements are being made and NiFe is becoming a viable alternative to lead acid in off-grid power systems. The battery is virtually immune to over- and under-charging and should last for over 50 years compared to less than 12 years with deep cycle lead acids in cycling mode. Pocket plate technology further lowered the self-discharge but NiFe costs about four-times as much as lead acid and is comparable with Li-ion.
The capacity of nickel-iron batteries can be improved by applying a high discharge current of up to three times the C-rate for periods of 30 minutes. Assure that the temperature of the electrolyte does not exceed 46˚C (115˚F).
Nickel-iron batteries use a taper charge similar to NiCd and NiMH. Allow the voltage to float; do not use constant voltage charge as applied to charging lead acid and lithium-ion batteries. Similar to nickel-based batteries, the cell voltage drops as gassing begins and the temperature rises. This can lead to an increase in current draw. Avoid overcharge as this causes water evaporation in the electrolyte.
A faded capacity of nickel-iron batteries can often be improved by applying a high discharge current of up to three times the C-rate for periods of 30 minutes. Assure that the temperature of the electrolyte does not exceed 46˚C (115˚F).
Nickel-zinc batteries are similar to nickel-cadmium in that they use an alkaline electrolyte and a nickel electrode, but differ in voltage; NiZn provides 1.65V/cell rather than 1.20V, which NiCd delivers.
In 1901, Thomas Alva Edison was awarded U.S. Patent for a rechargeable nickel–zinc battery system and installed in rail cars between 1932 and 1948. NiZn suffered from high self-discharge and short cycle life caused by dendrite growth, leading to electrical shorting. Improvements in the electrolyte have reduced this problem, and NiZn is being considered again for commercial uses. Low cost, high power output and good temperature operating range make this chemistry attractive.
NiZn charges at a constant current to 1.9V/cell and cannot take trickle charge, also known as maintenance charge. The specific energy is similar to other nickel-based systems. NiZn can by cycled 200–300 times, has no heavy toxic materials and can easily be recycled. Some are available in AA cells.
When research for nickel-metal-hydride began in 1967, problems with metal instabilities caused a shift towards the development of the nickel-hydrogen battery (NiH). NiH uses a steel canister to store hydrogen gas at a pressure of 1,200psi (8,270kPa). The cell includes solid nickel electrodes, hydrogen electrodes, gas screens and electrolyte that are encapsulated in the pressurized vessel.
NiH has a nominal cell voltage of 1.25V and the specific energy is 40–75Wh/kg. The advantages are long service life, even with full discharge cycles, good calendar life due to low corrosion, minimal self-discharge, and a remarkable temperature performance of –28°C to 54°C (–20°F to 130°F). These attributes make NiH ideal for satellite use. Scientists are developing NiH batteries for terrestrial use but low specific energy and high cost work against this endeavor. A single cell for a satellite application costs thousands of dollars. Satellite batteries are being replaced with long-life lithium ion.
Last updated 2015-05-28
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