Nickel–metal hydride battery

Not to be confused with a nickelâ€"hydrogen battery.

A nickelâ€"metal hydride battery, abbreviated NiMH or Niâ€"MH, is a type of rechargeable battery. The chemical reaction at the positive electrode (cathode) is similar to that of the nickelâ€"cadmium cell (NiCd), with both using nickel oxyhydroxide (NiOOH). However, the negative electrodes use a hydrogen-absorbing alloy instead of cadmium. A NiMH battery can have two to three times the capacity of an equivalent size NiCd, and its energy density can approach that of a lithium-ion battery.

Characteristics

The typical specific energy for small NiMH cells is about 100 Wh/kg. Larger NiMH cells achieve about 75 Wh/kg (270 kJ/kg). This is significantly better than the typical 40â€"60 Wh/kg for NiCd, and similar to the 100â€"160 Wh/kg for lithium-ion batteries. NiMH has a volumetric energy density of about 300 Wh/L (1,080 MJ/m³), significantly better than NiCd at 50â€"150 Wh/L, and about the same as lithium-ion at 250â€"360 Wh/L.

History

Work on NiMH batteries began at the Battelle-Geneva Research Center following the technology's invention in 1967. It was based on sintered Ti₂Ni+TiNi+x alloys and NiOOH-electrodes. Development was sponsored over nearly two decades by Daimler-Benz and by Volkswagen AG within Deutsche Automobilgesellschaft, now a subsidiary of Daimler AG. The batteries' specific energy reached 50 W·h/kg (180 kJ/kg), power density up to 1000 W/kg and a life of 500 charge cycles (at 100% depth of discharge). Patent applications were filed in European countries (priority: Switzerland), the United States, and Japan. The patents transferred to Daimler-Benz.

Interest grew in the 1970s with the commercialisation of the nickelâ€"hydrogen battery for satellite applications. Hydride technology promised an alternative, less bulky way to store the hydrogen. Research carried out by Philips Laboratories and France's CNRS developed new high-energy hybrid alloys incorporating rare earth metals for the negative electrode. However, these suffered from alloy instability in alkaline electrolyte and consequently insufficient cycle life. In 1987, Willems and Buschow demonstrated a successful battery based on this approach (using a mixture of La_0.8Nd_0.2Ni_2.5Co_2.4Si_0.1) which kept 84% of its charge capacity after 4000 chargeâ€"discharge cycles. More economically viable alloys using mischmetal instead of lanthanum were soon developed. Modern NiMH cells were based on this design. The first consumer grade NiMH cells became commercially available in 1989.

Ovonic Battery Co. in Michigan altered and improved the Tiâ€"Ni alloy structure and composition according to their patent and licensed NiMH batteries to over 50 companies. Ovonic's NiMH variation consisted of special alloys with disordered alloy structure and specific multicomponent alloy compositions. Unfortunately, due to their composition, the calendar and cycle life of such alloys remains low. All NiMH batteries manufactured at the present time consist of AB₅-type rare earth metal alloys.

A high-energy pasted electrode developed by Dr. Masahiko Oshitani from GS Yuasa Company led to the NiMH cell.

In 2008, more than 2 million hybrid cars worldwide were manufactured with NiMH batteries.

In the European Union and due to its Battery Directive, nickelâ€"metal hydride batteries replaced Niâ€"Cd batteries for portable consumer use.

About 22% of portable rechargeable batteries sold in Japan in 2010 were NiMH. In Switzerland in 2009, the equivalent statistic was approximately 60%. This percentage has fallen over time due to the increase in manufacture of lithium-ion batteries: in 2000, almost half of all portable rechargeable batteries sold in Japan were NiMH.

In 2015 BASF produced a modified microstructure that helped make NiMH batteries more durable, in turn allowing changes to the cell design that saved considerable weight, enabling storage of 140 watt-hours per kilogram.

Electrochemistry

The negative electrode reaction occurring in a NiMH cell is:

H₂O + M + e^âˆ' OH^âˆ' + MH

The charge reaction is read left-to-right and the discharge reaction is read right-to-left.

On the positive electrode, nickel oxyhydroxide, NiO(OH), is formed:

Ni(OH)₂ + OH^âˆ' NiO(OH) + H₂O + e^âˆ'

The metal M in the negative electrode of a NiMH cell is an intermetallic compound. Many different compounds have been developed for this application, but those in current use fall into two classes. The most common is AB₅, where A is a rare earth mixture of lanthanum, cerium, neodymium, praseodymium and B is nickel, cobalt, manganese, and/or aluminium. Some cells use higher-capacity negative electrode materials based on AB₂ compounds, where A is titanium and/or vanadium and B is zirconium or nickel, modified with chromium, cobalt, iron, and/or manganese, due to reduced lifetime. Any of these compounds serve the same role, reversibly forming a mixture of metal hydride compounds.

When overcharged at low rates, oxygen produced at the positive electrode passes through the separator and recombines at the surface of the negative. Hydrogen evolution is suppressed and the charging energy is converted to heat. This process allows NiMH cells to remain sealed in normal operation and to be maintenance-free.

NiMH cells have an alkaline electrolyte, usually potassium hydroxide. The positive electrode is nickel hydroxide and the negative electrode is hydrogen ions or protons. The hydrogen ions are stored in a metal hydride structure that is the electrode. For separation hydrophilic polyolefin nonwovens are used.

Charge

Charging voltage is in the range of 1.4â€"1.6 V/cell. In general, a constant-voltage charging method cannot be used for automatic charging. When fast-charging, it is advisable to charge the NiMH cells with a smart battery charger to avoid overcharging, which can damage cells. A NiCd charger is not a substitute for an automatic NiMH charger.

Trickle charging

The simplest, safe charging method is with a fixed low current, with or without a timer. Most manufacturers claim that overcharging is safe at very low currents, below 0.1 C (C/10) (where C is the current equivalent to the capacity of the battery divided by one hour). The Panasonic NiMH charging manual warns that overcharging for long enough can damage a battery and suggests limiting the total charging time to 10 to 20 hours.

Duracell further suggests that a trickle charge at C/300 can be used for batteries that must be kept in a fully charged state. Some chargers do this after the charge cycle, to offset natural self-discharge. A similar approach is suggested by Energizer, which indicates that self-catalysis can recombine gas formed at the anode for charge rates up to C/10. This leads to cell heating. The company recommends C/30 or C/40 for indefinite applications where long life is important. This is the approach taken in emergency lighting applications (which in Europe must last 4 hours) where the design remains essentially the same as in older NiCd units, except for an increase in the trickle charging resistor value. In comparison, NiCd cells can ordinarily be charged indefinitely at C/10 without damage.

Panasonic's handbook recommends that NiMH batteries on standby be charged by a lower duty cycle approach, where a pulse of a higher current is used whenever the battery's voltage drops below 1.3 V. This can extend battery life and use less energy.

Î"V charging method

In order to decrease charging time, the charger must stop charging before damaging the battery. One method is to monitor the change of voltage with time. As can be seen in the charge curve diagram, when the battery is fully charged the voltage across its terminals drops slightly. The charger can detect this and stop charging. This method is often used with nickelâ€"cadmium cells which display a large voltage drop at full charge. However, the voltage drop is much less pronounced for NiMH and can be non-existent at low charge rates, which can make the approach unreliable.

Another option is to monitor the change of voltage with respect to time and stop when this becomes zero, but this risks premature cutoffs. With this method, a much higher charging rate can be used than with a trickle charge, up to 1 C. At this charge rate, the voltage drop is approximately 5â€"10 mV per cell. Since this method measures the voltage across the battery, a constant current (rather than a constant voltage) charging circuit is used. This is unlike a leadâ€"acid cell for example, which can, in theory, be more easily charged at a suitably chosen constant voltage.

Î"T temperature charging method

The temperature change method is similar in principle to the Î"V method. Because the charging voltage is nearly constant, constant-current charging delivers energy at a near-constant rate. When the cell is not fully charged, most of this energy is converted to chemical energy. However, when the cell reaches full charge, most of the charging energy is converted to heat. This increases the rate of change of battery temperature, which can be detected by a sensor such as a thermistor. Both Panasonic and Duracell suggest a maximum rate of temperature increase of 1 °C per minute. Using a temperature sensor allows an absolute temperature cutoff, which Duracell suggests at 60 °C.

With both the Î"T and the Î"V charging methods, both manufacturers recommend a further period of trickle charging to follow the initial rapid charge.

Ambient temperature

Charging temperature limits for NiMH are stricter than operational limits. Nickel-based batteries are most forgiving in accepting charge at low temperatures, however, when charging below 5 °C (41 °F), the ability to recombine oxygen and hydrogen diminishes. If NiCd and NiMH are charged too rapidly, pressure builds up in the cell that leads to venting. Not only do escaping gases deplete the electrolyte, the hydrogen released is highly flammable. The charge current of all nickel-based batteries should be reduced to 0.1C below freezing down to âˆ'18 °C.

To enable fast-charging at all temperatures, some industrial batteries include a thermal blanket that heats the battery to an acceptable temperature; other chargers adjust the charge rate to prevailing temperatures. Consumer chargers do not have such facilities and operate at moderate temperature.

Safety

A resettable fuse in series with the cell, particularly of the bimetallic strip type, increases safety. This fuse opens if either the current or the temperature goes too high.

Modern NiMH cells contain catalysts to handle gases produced by over-charging (2 H₂ + O₂ â€" catalyst â†' 2 H₂O). However, this only works with overcharging currents of up to 0.1 C (nominal capacity divided by ten hours). This reaction causes batteries to heat, ending the charging process. Some quick chargers have a cooling fan.

A method for very rapid charging called in-cell charge control involves an internal pressure switch in the cell, which disconnects the charging current in the event of overpressure.

One inherent risk with NiMH chemistry is that overcharging causes hydrogen buildup, potentially rupturing the cell. Therefore, cells have a vent to release the gas in the event of serious overcharging.

Nickel metal hydride batteries are made of environmentally friendly materials. The batteries contain only mild toxins and are recyclable.

Failure modes

NiMH batteries predominantly fail in two somewhat related modes. The metal hydride material used for the negative electrode undergoes gradual corrosion in a strong alkaline environment. This corrosion results in less active material for hydrogen storage and also consumes water from the electrolyte.

This results in a gradual loss of power as water is consumed increasing the cell resistance, with a gradual loss in capacity as active material is converted to corrosion products. By optimization of the alloy composition, this corrosion process can be reduced. The corrosion rate is influenced by factors including temperature, State of Charge (SoC), and the control of overcharge and oxygen recombination. Studies under controlled overcharge conditions predict that the service life is halved for each approximately 20 °C.(36 °F.) rise in temperature. Thus, a battery designed to operate for 20 years at 25 °C (77 °F) would last 10 years at 45 °C (113 °F). Extrapolation beyond 45 °C is not linear since other failure modes, caused by decreasing charge acceptance resulting in positive electrode swelling and thermal instability, control the battery life.

Discharge

A fully charged cell supplies an average 1.25 V/cell during discharge, declining to about 1.0â€"1.1 V/cell (further discharge may cause permanent damage in the case of multi-cell packs, due to polarity reversal). Under a light load (0.5 ampere), the starting voltage of a freshly charged AA NiMH cell in good condition is about 1.4 volts.

Over-discharge

Complete discharge can reverse polarity in one or more cells, which can permanently damage them. This situation can occur in the common arrangement of four AA cells in series in a digital camera, where one completely discharges before the others due to small differences in capacity among the cells. When this happens, the good cells start to drive the discharged cell in reverse. Some cameras, GPS receivers and PDAs detect the safe end-of-discharge voltage of the series cells and auto-shutdown, but devices such as flashlights and some toys do not. A single cell driving a load or a cell connected in parallel to other cells cannot suffer from polarity reversal, because no other cells are present. (Cells parallel-connected to a discharged cell tend to forward-charge it.)

Irreversible damage from polarity reversal is a particular danger, even when a low voltage threshold cutout is employed, should the cells vary in temperature. This is because capacity significantly declines as the cells are cooled. This results in a lower voltage under load of the colder cells.

Self-discharge

NiMH cells historically had a somewhat higher self-discharge rate (equivalent to internal leakage) than NiCd cells. The self-discharge rate varies greatly with temperature, where lower storage temperature leads to slower discharge rate and longer battery life. The self-discharge is 5â€"20% on the first day and stabilizes around 0.5â€"4% per day at room temperature. But at 45 °C it is approximately 3 times as high.This is not a problem in the short term but makes them unsuitable for many light-duty uses, such as clocks, remote controls, or safety devices, where the battery would normally be expected to last many months or years. The highest capacity cells on the market (>8000 mA·h) are reported to have the highest self-discharge rates.

Reduced self-discharge

The low self-discharge nickelâ€"metal hydride battery(LSD NiMH) was introduced in 2005 by Sanyo. It has a significantly lower rate of self-discharge. By using an improved electrode separator and improved positive electrode, manufacturers claim the cells retain 70% to 85% of their capacity when stored one year at 20 °C (68 °F), compared to about half for normal NiMH batteries. They are otherwise similar to other NiMH batteries, and can be charged in the typical chargers. These cells are marketed as "hybrid", "ready-to-use" or "pre-charged" rechargeables. Retention of charge depends a lot on the battery's impedance or internal resistance (the lower the better), and on physical size and charge capacity.

Separators keep the two electrodes apart to slow electrical discharge while allowing the transport of ionic charge carriers that close the circuit during the passage of current. High quality separators are critical for battery performance.

Thick separators are one way to reduce self-discharge, but take up space and reduce capacity; while thin separators tend to raise the self-discharge rate. Some batteries may have overcome this tradeoff using thin separators with more precise manufacturing and by using a more advanced sulfonated polyolefin separator.

Low self-discharge cells have lower capacity than standard NiMH cells because of the separator's larger volume. The highest-capacity low-self-discharge AA cells have 2000â€"2600 mA·h capacity, and AAA 1000 mA·h, compared to 2800 mA·h and 1300 mA·h for high-capacity AA and AAA NiMH cells.

Most manufacturers produce only size AAA and AA batteries, making up most of the LSD market. Larger size C and D cells are available, though some are actually AA cells inside a C/D-sized case.

Comparison with other battery types

NiMH cells are often used in digital cameras and other high drain devices, where over the duration of single charge use they outperform primary (such as alkaline) batteries.

NiMH cells are advantageous for high current drain applications, largely due to their lower internal resistance. Alkaline batteries, which offer approximately 3000 mA·h capacity at low current demand (200 mA), provide only 700 mA·h capacity with a 1000 mA load. Digital cameras with LCDs and flashlights can draw over 1000 mA, quickly depleting them. NiMH cells can deliver these current levels without similar loss of capacity.

Certain devices that were designed to operate using primary alkaline chemistry (or zincâ€"carbon/chloride) cells will not function with NiMH cells. However most devices compensate for the voltage drop of an alkaline battery as it discharges down to about 1 volt. Low internal resistance allows NiMH cells to deliver a near-constant voltage until they are almost completely discharged. Battery level indicators overstate the remaining charge if it was designed to read alkaline cells. The voltage of alkaline cells decreases steadily during most of the discharge cycle.

Lithium-ion batteries have a higher specific energy than nickelâ€"metal hydride batteries, but they are significantly more expensive. Each nickel metal hydride battery costs about $60 with each cycle costing only $0.12.

Leadâ€"acid batteries are very temperature sensitive. Grid corrosion and water loss accelerate as battery temperature increases. The service life is halved with each 8 °C.(14.4 °F.) rise. Thus, a battery designed to operate for 10 years at 25 °C (77 °F) would last approximately only 5 years at 33 °C (91 °F), and only 2.5 years at 41 °C (106 °F).

NiCd batteries fail in a different manner. They are susceptible to failures caused by short circuiting due to dissolution/crystallization reactions occurring at the negative electrode, which can result in Cd dendrite growth creating a short to the positive plate.

As of 2005 nickel metal hydride batteries constituted three percent of the battery market.

Applications

Consumer electronics

NiMH batteries have replaced NiCd for many roles, notably small rechargeable batteries. NiMH batteries are commonly used for AA (penlight-size) batteries. These have nominal charge capacities (C) of 1.1â€"2.8 Ah at 1.2 V, measured at the rate that discharges the cell in five hours. Useful discharge capacity is a decreasing function of the discharge rate, but up to a rate of around 1×C (full discharge in one hour), it does not differ significantly from the nominal capacity. NiMH batteries nominally operate at 1.2 V per cell, somewhat lower than conventional 1.5 V cells, but will operate many devices designed for that voltage.

NiMH technology is used in iRobot's Roomba robotic vacuum cleaners as well as the humanoid prototype robot ASIMO designed by Honda. NiMH batteries are commonly used in remote control cars.

Electric vehicles

Applications of NiMH electric vehicle batteries include all-electric plug-in vehicles such as the General Motors EV1, Honda EV Plus, Ford Ranger EV and Vectrix scooter. Hybrid vehicles such as the Toyota Prius, Honda Insight, Ford Escape Hybrid, Chevrolet Malibu Hybrid and Honda Civic Hybrid also use them.

Stanford R. Ovshinsky invented and patented a popular improvement of the NiMH battery and founded Ovonic Battery Company in 1982. General Motors purchased Ovonics' patent in 1994. By the late 1990s, NiMH batteries were being used successfully in many fully electric vehicles, such as the General Motors EV1 and Dodge Caravan EPIC minivan. In October 2000, the patent was sold to Texaco and a week later Texaco was acquired by Chevron. Chevron's Cobasys subsidiary provides these batteries only to large OEM orders. General Motors shut down production of the EV1 citing lack of battery availability as a chief obstacle. Cobasys control of NiMH batteries created a patent encumbrance for large automotive NiMH batteries. - NiMH batteries are used on the Alstom Citadis low floor tram ordered for Nice, France. As of 2015 (actually much earlier) all of the relevant patents have expired or were not awarded or maintained, hence there are no extant patent encumbrances concerning the use of NiMH batteries in electric cars.

Telecommunications

In recent years, NiMH batteries have been deployed to provide backup power for telecommunications service providers.

They are marketed for use in the Central Office (CO), Outside Plant (OSP), and at locations such as Controlled Environmental Vaults (CEVs), Electronic Equipment Enclosures (EEEs), huts and in uncontrolled structures such as cabinets.

To ensure conformance and product safety, the telecommunications industry devised a three-level compliance system, as described in GR-3168, Generic Requirements for Nickel Metal Hydride (NiMH) Battery Systems for Telecommunications Use. The system provides a common framework for evaluating and qualifying NiMH battery technologies.

Service life is extremely important in a telecommunications environment.

See also

• Battery recycling
• Chevron Corporation
• Electric car
• Gas diffusion electrode
• Lithium-ion battery
• Nickelâ€"zinc battery
• Nickelâ€"hydrogen battery
• Nickel(II) hydroxide
• Nickel(III) oxide
• Patent encumbrance of large automotive NiMH batteries
• Power-to-weight ratio

References

External links

• "Bipolar Nickel Metal Hydride Battery" by Martin G. Klein, Michael Eskra, Robert Plivelich and Paula Ralston
• Energizer Nickel Metal Hydride (NiMH) Handbook and Application Manual
• Chevron/Texaco's patent on the NiMH battery
• NiMH battery charging and safety

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