BU-203: Nickel-based Batteries

For 50 years, portable devices relied almost exclusively on nickel-cadmium (NiCd). This generated a large amount of data, but in the s, nickel-metal-hydride (NiMH) took over the reign to solve the toxicity problem of the otherwise robust NiCd. Many of the characteristics of NiCd were transferred to the NiMH camp, offering a quasi-replacement as these two systems are similar. Because of environmental regulations, NiCd is limited to specialty applications today.

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Nickel-cadmium (NiCd)

Invented by Waldemar Jungner in , the nickel-cadmium battery offered several advantages over lead acid, then the only other rechargeable battery; however, the materials for NiCd were expensive. Developments were slow, but in , advancements were made to deposit the active materials inside a porous nickel-plated electrode. Further improvements occurred in by absorbing the gases generated during charge, which led to the modern sealed NiCd battery.

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 s, the ultra-high capacity NiCd rocked the world with capacities that were up to 60 percent higher than the standard NiCd. Packing more active material into the cell achieved this, but the gain was shadowed by higher internal resistance and reduced cycle count.

The standard NiCd remains one of the most rugged and forgiving batteries, and the airline industry stays true to this system, 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)

According to RWTH, Aachen, Germany (), the cost of NiCd is about $400 per kWh[1]. Table 1 lists the advantages and limitations of the standard NiCd.

• Rugged, high cycle count with proper maintenance
• Only battery that can be ultra-fast charged with little stress
• Good load performance; forgiving if abused
• Long shelf life; can be stored in a discharged state, needs priming before use
• 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 and can be rejuvenated
  
• Cadmium is a toxic metal. Cannot be disposed of in landfills
• High self-discharge; needs recharging after storage
• Low cell voltage of 1.20V requires many cells to achieve high voltage

Nickel-metal-hydride (NiMH)

Research on nickel-metal-hydride started in ; however, instabilities with the metal-hydride led to the development of the nickel-hydrogen (NiH) instead. New hydride alloys discovered in the s eventually improved the stability issues and today NiMH provides 40 percent higher specific energy than the standard NiCd

Nickel-metal-hydride is not without drawbacks. The battery is more delicate and trickier to charge than NiCd. With 20 percent self-discharge in the first 24 hours after charge and 10 percent per month thereafter, NiMH ranks among the highest in the class. 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 long life span.

Consumer Applications

NiMH has become one of the most readily available rechargeable batteries for consumer use. Battery manufacturers, such as Panasonic, Energizer, Duracell and Rayovac, have recognized the need for a durable and low-cost rechargeable battery and offer NiMH in AA, AAA and other sizes. The battery manufacturers want to lure buyers away from disposable alkaline to rechargeable batteries.

The NiMH battery for the consumer market is an alternative for the failed reusable alkaline that appeared in the s. 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 cells and to always use the same type of batteries in the holder. Safety concerns and voltage incompatibility prevent the sale of most lithium-ion batteries in AA and AAA formats.

* Eneloop is a Panasonic () trademark, based on NiMH.
** Self-discharge is highest right after charge, and then tapers off.

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 with many consumers especially for flashlights that sit on standby for the occasional power-outage; alkaline keeps the charge for 10 years.

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.

Table 3 summarizes the advantages and limitations of industrial-grade NiMH. The table does not include the Eneloop and other consumer brands.

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• 30&#;40 percent higher capacity than a standard NiCd
• Less prone to memory than NiCd, can be rejuvenated
• Simple storage and transportation; not subject to regulatory control
• Environmentally friendly; contains only mild toxins
• Nickel content makes recycling profitable
• Wide temperature range

• Limited service life; deep discharge reduces service life
• Requires complex charge algorithm. Sensitive to overcharge
  
• Does not absorb overcharge well; trickle charge must be kept low
• Generates heat during fast charge and high-load discharge
• High self-discharge
• Coulombic efficiency only about 65% (99% with Li-ion)

Nickel-iron (NiFe)

After inventing nickel-cadmium in , Sweden&#;s Waldemar Jungner tried to substitute cadmium for iron to save money; however, poor charge efficiency and gassing (hydrogen formation) prompted him to abandon the development without securing a patent.

In , Thomas Edison continued the development of the nickel-iron battery as a substitute to lead acid for electric vehicles. He claimed that nickel-iron, immersed in an alkaline electrolyte, was &#;far superior to batteries using lead plates in sulfuric acid.&#; He counted on the emerging electric vehicle market and lost out when gasoline-powered cars took over. His disappointment grew when the auto industry used lead acid as the battery for starter, lighting and ignition (SLI) instead of nickel-iron. (See BU-: Electric Powertrain, HEV, PHEV)

Edison promoted Nickel-iron as being lighter and cleaner than lead acid. Lower operational costs were to offset the higher initial cost. In ca. Edison recognized the need for the electric car. He said that the same care should be given to the battery as the horse and railroad locomotive.

The nickel-iron battery (NiFe) uses an oxide-hydroxide cathode and an iron anode with potassium hydroxide electrolyte that produces a nominal cell voltage of 1.20V. 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; during World War II the battery powered German V-1 flying bombs and the V-2 rockets. Other uses are railroad signaling, forklifts and stationary applications.

NiFe has a low specific energy of about 50Wh/kg, has poor low-temperature performance and exhibits high self-discharge of 20&#;40 percent a month. This, together 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. Pocket plate technology lowered the self-discharge; the battery is virtually immune to over- and under-charging and should last for over 50 years. This compares to less than 12 years with deep cycle lead acids in cycling mode. NiFe costs about four times as much as lead acid and is comparable with Li-ion in purchase price.

Nickel-iron batteries use a taper charge similar to NiCd and NiMH. Do not use constant voltage charge as with lead acid and lithium-ion batteries, but allow the voltage to float freely. Similar to nickel-based batteries, the cell voltage begins to drop at full charge as the internal gas builds up and the temperature rises. Avoid overcharging as this causes water evaporation and dry-out. Only trickle charge to compensate self-discharge.

Low capacity 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 (NiZn)

Nickel-zinc is similar to nickel-cadmium in that it uses an alkaline electrolyte and a nickel electrode, but it differs in voltage; NiZn provides 1.65V/cell rather than 1.20V, which NiCd and NiMH deliver. NiZn charges at a constant current to 1.9V/cell and cannot take trickle charge, also known as maintenance charge. The specific energy is 100Wh/kg and can be cycled 200&#;300 times. NiZn has no heavy toxic materials and can easily be recycled. Some packaging is available in the AA cell format.

In , Thomas Edison was awarded the U.S. patent for a rechargeable nickel&#;zinc battery system that was installed in rail cars between and . NiZn suffered from high self-discharge and short cycle life caused by dendrite growth, which often led to an electrical short. 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.

Nickel-hydrogen (NiH)

When research for nickel-metal-hydride began in , problems with metal instabilities caused a shift towards the development of the nickel-hydrogen battery (NiH). NiH uses a steel canister to store hydrogen at a pressure of 8,270kPa (1,200psi). 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 tried to develop NiH batteries for terrestrial use, but low specific energy and high cost worked against this endeavor. A single cell for a satellite application costs thousands of dollars. As NiH replaced NiCd in satellites, there is a move towards long-life Li-ion. (See BU-211: Alternate Battery Systems)

References

[1] RWTH, Aachen
[2] "Thomas A. Edison & His Improved Storage Battery." SCIENTIFIC AMERICAN, January : Front Page.

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