- Nickel–metal hydride battery
Nickel–metal hydride battery
Modern, high capacity NiMH rechargeable cells
specific energy 60–120 W·h/kg energy density 140–300 W·h/L specific power 250–1000 W/kg Charge/discharge efficiency 66% Energy/consumer-price 2.75 W·h/US$ Self-discharge rate 30%(2%)/month (temperature dependent) Cycle durability 500–1,000 Nominal cell voltage 1.2 V
A nickel–metal hydride cell, abbreviated NiMH, is a type of rechargeable battery similar to the nickel–cadmium cell. The NiMH battery uses a hydrogen-absorbing alloy for the negative electrode instead of cadmium. As in NiCd cells, the positive electrode is nickel oxyhydroxide (NiOOH). A NiMH battery can have two to three times the capacity of an equivalent size nickel–cadmium battery. The energy density of a NiMH cell is similar to that of a lithium-ion cell, but the rate of self-discharge is higher. This means that a stored NiMH battery will lose charge more quickly. In 2005 a low self-discharge NiMH battery (LSD), which stays charged for much longer, was developed.
Common AA (penlight-size) NiMH batteries have nominal charge capacities (C) ranging from 1100 mA·h to 3100 mA·h 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.
The typical specific energy for NiMH AA cells is about 100 W·h/kg, and for other NiMH dry cells about 75 W·h/kg (270 kJ/kg), compared to 40–60 W·h/kg for Ni–Cd, or 100-160 W·h/kg for Li-ion. NiMH has a volumetric energy density of about 300 W·h/L (1080 MJ/m³), significantly better than nickel–cadmium at 50–150 W·h/L, and about the same as Li-ion at 250-360 W·h/L.
About 22% of portable rechargeable batteries sold in Japan in 2010 were nickel–metal hydride. 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 nickel–metal hydride.
Because non-LSDs do not have an LSD separator, they are cheaper to manufacture than LSDs, yet most are offered at about the same price as LSDs and are marketed as "high capacity" or "ultra high capacity" NiMH batteries. While "high capacity" versions may have an extra 20% in initial capacity (compared to LSDs), this is negated by much higher internal resistance (especially in high drain situations) than LSDs and much higher self discharge rates (20% or more in first 24 hours, plus 4% per day thereafter). This energy wasted on heat and self discharge means these batteries require significant extra recharging which reduces overall battery life. Consequently, with consumer size batteries (AAA, AA, C, D, 9v), the low self-discharge NiMH battery has all but replaced the "high capacity" or "ultra high capacity" non-LSD type. 
- 1 History
- 2 Applications
- 3 Electrochemistry
- 4 Charging
- 5 Discharging
- 6 Environmental impact
- 7 Comparison with other battery types
- 8 Patent encumbrance in electric vehicles
- 9 See also
- 10 References
- 11 External links
The first consumer grade NiMH cells for smaller applications appeared on the market in 1989, the culmination of over two decades of research and development.
The earliest pioneering work on NiMH batteries — essentially based on sintered Ti2Ni+TiNi+x alloys for the negative electrode and NiOOH-electrodes for the positives — was performed at the Battelle-Geneva Research Center starting after its invention in 1967. The development work was sponsored over nearly two decades by Daimler-Benz in Stuttgart, Germany, and by Volkswagen AG within the framework of Deutsche Automobilgesellschaft, now a subsidiary of Daimler AG. The batteries showed high specific energy up to 50 W·h/kg (180 kJ/kg), power density up to 1000 W/kg and a reasonable life of 500 charge cycles (at 100% depth of discharge). Patent applications were filed in European countries (priority: Switzerland), United States and Japan and the patents transferred to Daimler-Benz.
Interest grew in the 1970s with the commercialisation of the Nickel hydrogen battery for satellite applications. Hydride technology promised an alternative much 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 the instability of the alloys 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 La0.8Nd0.2Ni2.5Co2.4Si0.1) which kept 84% of its charge capacity after 4000 charge-discharge cycles. More economically viable alloys using mischmetal instead of lanthanum were soon developed and modern NiMH cells are based on this design.
Ovonic Battery Co. in Michigan altered and improved the Ti-Ni alloy structure and composition according to their patent and licensed NiMH batteries to over 50 companies worldwide. Ovonic's NiMH variation consisted of special alloys with disordered alloy structure and specific multicomponent alloy compositions. Unfortunately, linked to their composition, the calendar and cycle life of such alloys always remains very low, and all NiMH batteries manufactured at the present time consist of AB5-type rare earth metal alloys.
Positive electrode development was done by Dr. Masahiko Oshitani from GS Yuasa Company, who was the first to develop high-energy paste electrode technology. The association of this high-energy electrode with high-energy hybrid alloys for the negative electrode led to the new environmentally friendly high-energy NiMH cell.
Currently, more than 2 million hybrid cars worldwide are running with NiMH batteries, e.g., Prius, Lexus (Toyota), Civic, Insight (Honda), Fusion (Ford), and others. Many of these batteries are manufactured by PEVE (Panasonic) and Sanyo.
Applications of NiMH electric vehicle batteries includes 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. NiMH technology is used extensively in rechargeable batteries for consumer electronics, and it will also be used on the Alstom Citadis low floor tram ordered for Nice, France; as well as the humanoid prototype robot ASIMO designed by Honda. NiMH batteries are also commonly used in remote control cars.
The negative electrode reaction occurring in a NiMH cell is
The charge reaction is read left-to-right and the discharge reaction is read right-to-left.
On the positive electrode, nickel oxyhydroxide (NiOOH) is formed,
The "metal" M in the negative electrode of a NiMH cell is actually 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. Very few cells use higher-capacity negative material electrodes 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 the reduced life performances. 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.
The charging voltage is in the range of 1.4–1.6 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 and even be dangerous. A NiCd charger should not be used as an automatic substitute for a NiMH charger.
The simplest way to safely charge a NiMH cell 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 (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, for applications where the battery must be kept in a fully charged state, a trickle charge at 0.0033 C can be used. Some chargers do this after the charge cycle, to offset the natural self-discharge rate of the battery. Panasonic's handbook, however, recommends that such batteries are kept 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.
ΔV charging method
In order to charge a NiMH battery faster than the trickle method suggested above, a charger must know when to stop charging in order to avoid damaging the battery. One method is to monitor the change of voltage across the battery 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 Nickel-Cadmium cells which have a large drop in voltage at full charge but the voltage drop is much less pronounced for NiMH and can be non-existent at high 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 runs the risk of 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, ΔV is approximately 5–10mV per cell. Since this method measures the voltage across the battery, a constant current (rather than a constant voltage) charging circuit must be used. This is unlike a lead-acid cell for example, which can, in theory, be more easily charged at a suitably chosen constant voltage.
ΔT temperature charging method
The ΔT temperature change method is similar in principle to the ΔV 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 also allows an absolute temperature cutoff, which Duracell suggests at 60°C.
With both the ΔT and the ΔV charging methods, both manufacturers recommend a further period of trickle charging to follow the initial rapid charge.
A good safety feature of a custom-built charger is to use a resettable fuse in series with the cell, particularly of the bimetallic strip type. This fuse will open if either the current or the temperature goes too high.
Modern NiMH cells contain catalysts to immediately deal with gases developed as a result of over-charging without being harmed (2 H2 + O2 ---catalyst → 2 H2O). However, this only works with overcharging currents of up to 0.1C (nominal capacity divided by 10 hours). As a result of this reaction, the batteries will heat up considerably, marking the end of the charging process. Some quick chargers have a fan to keep the batteries cool.
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.
There is an inherent risk with NiMH chemistry that overcharging will cause a buildup of hydrogen, causing the cell to rupture. Therefore, cells have a vent. Hydrogen will be emitted from the vent in the event of serious overcharging.
A fully charged cell measures 1.4–1.45 V (unloaded), and supplies a nominal average 1.25 V/cell during discharge, down to about 1.0–1.1 V/cell (further discharge may cause permanent damage in the case of multi-cell packs, due to polarity reversal). This voltage varies depending on the discharge rate of the cell (lower discharge loads result in an increased voltage output for longer periods, approaching the 1.4 V unloaded cell voltage).
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; some measure almost 1.5 volts. This voltage falls rapidly to about 1.25 volts at 10% depth of discharge (DOD) and then remains almost constant until the cell is over 80% discharged. The voltage then falls rapidly from about 1.2 volts down to 0.8–1.0 volts at which the cell is considered "flat" in most devices. Mid-discharge at a load of 1 ampere, the output is about 1.2 volts; at 2 amperes, about 1.15 volts; the total effective differential internal resistance is about 0.05 ohms. Nickel metal hydride batteries provide a relatively constant voltage for most of the discharge cycle, unlike a standard alkaline where the voltage falls steadily during discharge.
A complete discharge of a cell until it goes into polarity reversal can cause permanent damage to the cell. This situation can occur in the common arrangement of four AA cells in series in a digital camera, where one will be completely discharged before the others due to small differences in capacity among the cells. When this happens, the good cells will start to drive the discharged cell in reverse, which can cause permanent damage to that cell. Some cameras, GPS receivers and PDAs detect the safe end-of-discharge voltage of the series cells and auto-shutdown, but devices like flashlights and some toys do not. A single cell driving a load can't suffer from polarity reversal, because there are no other cells to reverse-charge it when it becomes discharged.
Irreversible damage from polarity reversal is a particular danger in systems, even when a low voltage threshold cutout is employed, where cells in the battery are of different temperatures. This is because the capacity of NiMH cells significantly declines as the cells are cooled. This results in a lower voltage under load of the colder cells.
NiMH cells historically had a somewhat higher self-discharge rate (equivalent to internal leakage) than NiCd cells. The self-discharge is 5–10% on the first day and stabilizes around 0.5–1% per day at room temperature. 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 rate is strongly affected by the temperature at which the batteries are stored with cooler storage temperatures leading to slower discharge rate and longer battery life. The highest capacity cells on the market (>8000 mA·h) are reported to have the highest self-discharge rates.
Low self-discharge cells
A new type of nickel-metal hydride cell was introduced in 2005 that reduces self-discharge and therefore lengthens shelf life. By using a new separator, manufacturers claim the cells retain 70% to 85% of their capacity after one year when stored at 20 °C (68 °F). These cells are marketed as "hybrid", "ready-to-use" or "pre-charged" rechargeables. Besides the longer shelf life, they are otherwise similar to normal NiMH batteries of equivalent capacity and can be charged in typical NiMH chargers.
Low self-discharge cells have lower capacity than some standard NiMH cells due to the larger area of the separator. The highest capacity low-self-discharge cells have 2000–2500 mA·h and 1000 mA·h capacities for AA and AAA cells respectively, compared to 2800 mA·h and 1300 mA·h for standard AA and AAA cells. C types are typically higher than their usual NiMH cousins, with 4000 mA·h and the D type being 8000 mA·h.
However, after only a few weeks of storage, the retained capacity of low-self-discharge batteries often exceeds that of traditional NiMH batteries of higher capacity.
Improper disposal of NiMH batteries poses less environmental hazard than that of NiCd because of the absence of toxic cadmium. However, mining and processing the various alternate metals that form the negative electrode may pose other types of environmental impact, depending on the metal, mining method, and environmental practices of the mine.
Comparison with other battery types
NiMH cells and chargers are readily available in retail stores in the common sizes AAA and AA. Adapter sleeves are available to use the more common AA size in C and D applications. The sizes C and D cells are somewhat available, but are often just a AA core hidden in an outer shell, with a rating of about 2500 mA·h, much less than ordinary alkaline C and D batteries. Real NiMH C and D batteries are expensive (and the chargers are uncommon); they should be rated at least 5000 mA·h for C and 10,000 mA·h for D sizes.
PP3 (nine volt) NiMH batteries are available; these usually have an output voltage of 8.4 V (1.2 × 7) and a capacity of roughly 200 mA·h. Also available are eight-cell nine volt batteries with a nominal output voltage of 9.6 V (1.2 × 8).
NiMH cells are not expensive, and the voltage and performance is similar to primary alkaline cells in those sizes; they can be substituted for most purposes. Although alkaline cells are rated at 1.5 volts and NiMH cells at 1.2 volts, during discharge the alkaline voltage eventually drops below that of NiMH. NiMH batteries offer a flatter discharge curve, particularly at higher current draw.
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. Applications that require frequent replacement of the battery, such as toys or video game controllers, also benefit from use of rechargeable batteries. With the development of low self-discharge NiMHs (see section above), many occasional-use and very low-power applications are now candidates for NiMH cells.
NiMH cells are particularly advantageous for high current drain applications, due in large part to their low internal resistance. Alkaline batteries, which might have approximately 3000 mA·h capacity at low current demand (200 mA), will have about 700 mA·h capacity with a 1000 mA load. Digital cameras with LCDs and flashlights can draw over 1000 mA, quickly depleting alkaline batteries. NiMH cells can deliver these current levels and maintain their full capacity.
Certain devices that were designed to operate using primary alkaline chemistry (or zinc-carbon/chloride) cells will not function when one uses NiMH cells as substitutes. However, this is rare, as most devices compensate for the voltage drop of an alkaline as it discharges down to about 1 volt. A good-quality freshly charged NiMH cell delivers 1.4–1.45 V, very close to the 1.5 V that these devices expect. Such devices would also likely have an extremely short runtime as the voltage from an alkaline falls to 1.4 V quite quickly from the 1.5 V starting voltage. Low internal resistance allows NiMH cells to deliver a near-constant voltage until they are almost completely discharged. This will cause a battery level indicator to overstate the remaining charge if it was designed to read only the voltage curve of alkaline cells. The voltage of alkaline cells decreases steadily during most of the discharge cycle.
Lithium ion batteries have a higher specific energy than nickel-metal hydride batteries, but they are significantly more expensive to produce. In October 2009, ECD Ovonics announced that their next-generation NiMH batteries will provide specific energy and power that are comparable to those of lithium ion batteries at a cost that is significantly lower than the cost of lithium ion batteries.
Patent encumbrance in electric vehicles
Stanford R. Ovshinsky invented and patented ( a popular improvement of ) the NiMH battery and founded Ovonic Battery Company in 1982. General Motors purchased the patent from Ovonics 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 will only provide these batteries to large OEM orders. General Motors shut down production of the EV1 citing lack of battery availability as one of their chief obstacles. The Cobasys control of NiMH batteries has created a patent encumbrance of large automotive NiMH batteries.
- Nickel–cadmium battery
- Nickel–hydrogen battery
- Chevron Corporation
- Low self-discharge NiMH battery
- Gas diffusion electrode
- Nickel(II) hydroxide
- Nickel(III) oxide
- Electric Car
- Patent encumbrance of large automotive NiMH batteries
- Power-to-weight ratio
- Battery recycling
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- "Bipolar Nickel Metal Hydride Battery" by Martin G. Klein, Michael Eskra, Robert Plivelich and Paula Ralston
- Sanyo Eneloop NiMH Rechargeable Batteries Review
- Choosing and Using NiMH Rechargeable Batteries
- Duracell Ni-MH Technical Bulletin
- Energizer Ni-MH Battery Datasheets
- Rayovac battery Specifications and Product Guides
- Brand Neutral Drawings of Nickel Metal Hydride Batteries
- Chevron/Texaco's patent on the NiMH battery
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