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Nickel metal hydride battery
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A nickel-metal hydride cell, abbreviated NiMH, is a type of secondary electrochemical cell similar to Nickel Hydrogen 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 cell can have two to three times the capacity of an equivalent size Nickel-cadmium battery. However, compared to the lithium-ion cell, the volumetric energy density is lower and self-discharge is higher.
Common AA cells (penlight-size) NiMH batteries have nominal charge capacities (C) ranging from 1100 mA·h to 2900 mA·h at 1.2 V, usually measured at a discharge rate of 0.2×C per hour.
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Encyclopedia
A nickel-metal hydride cell, abbreviated NiMH, is a type of secondary electrochemical cell similar to Nickel Hydrogen 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 cell can have two to three times the capacity of an equivalent size Nickel-cadmium battery. However, compared to the lithium-ion cell, the volumetric energy density is lower and self-discharge is higher.
Common AA cells (penlight-size) NiMH batteries have nominal charge capacities (C) ranging from 1100 mA·h to 2900 mA·h at 1.2 V, usually measured at a discharge rate of 0.2×C per hour. 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 specific energy density for NiMH material is approximately 70 W·h/kg (250 kJ/kg), with a volumetric energy density of about 300 W·h/L (360 MJ/m³).
History
The first consumer grade NiMH cells for smaller applications appeared on the market in 1989 as a variation of the 1970s nickel hydrogen cell. Positive electrode development was done by Dr. Masahiko Oshitani from 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, discovered by Philips Laboratories and French CNRS labs in the 1970s, led to the new environmentally-friendly high-energy NiMH cell.
Applications
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, 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.
Electrochemistry
The negative electrode reaction occurring in a NiMH cell is
The electrode is charged in the right direction of this equation and discharged in the left direction.
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 serves 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.
Charging
The charging voltage is in the range of 1.4–1.6 V/cell. A fully charged cell measures 1.35–1.4 V (unloaded), and supplies a nominal average 1.2 V/cell during discharge, down to about 1.0–1.1 V/cell (further discharge may cause permanent damage). 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 cause dangerous conditions. A Ni-Cad charger should not be used as an automatic substitute for a NiMH charger.
?V charging method
One of the preferred charging methods is the "?V " method, according to Panasonic and other cell manufacturers. This is illustrated in the "NiMH Charge curve" figure. The charger measures the rate of change (signified by the symbol ?) of the voltage of the cell (signified by the letter V). The cell or battery is rapidly charged at a constant current of 1 C/h, where C is the capacity of the battery (the current is expressed as ampere hours per hour). After the cell is fully charged, and as it begins to overcharge, the voltage polarity of the electrodes inside the battery will begin to reverse, and this will cause the battery voltage to decrease slightly. A "?V" type battery charger ends the charge cycle by switching off the charging current when it senses this drop in voltage. In some cases, a very small "trickle charge" may remain. The "charge curve" graph also shows that the charge voltage will change depending on the charge current. (Incidentally, it also changes with temperature and battery age.) This generally means that a constant-voltage charging method cannot be used automatically, because it will either be unsafe, or it will not charge batteries reliably and consistently. This is unlike a lead-acid cell for example, which can, in theory, be more easily charged at a suitably chosen constant voltage.
? temperature charging method
The ? temperature method is similar in principle to the ?V method. Because the charging voltage is nearly constant, if constant-current charging is used, then a near constant power is entering the cell. When the cell is charging, most of this power will be converted to chemical energy. However, when the cell is fully charged, most of the charging power will then be converted to heat. This results in an increase in the rate of change of temperature, which can be detected by a sensor measuring the battery temperature. This signal is monitored by the charger, which then stops the charging current.
Manual charging
If a suitable battery charger is not available, constant-voltage or constant-current charging can be done manually, at a moderately high charging rate, if careful attention is given. For proper charging, the voltage and/or current must be set to a suitable charging rate for the particular battery, and a timer should be set. Periodic monitoring is strongly recommended to avoid overcharging (resulting in a voltage drop), or overheating (resulting in an excessive temperature rise and possibly an overpressure condition).
Trickle charging
Some equipment manufacturers consider that NiMH cells can be safely charged in simple fixed, low-current chargers with or without timers, and that permanent overcharging is permissible with currents up to C/10 h. According to the Panasonic NiMH charging manual, extensive trickle charging can cause battery deterioration due to overcharging, and it is the least preferred charging method concerning battery performance. If it is used, the trickle charge rate should be limited to between 0.033×C per hour and 0.05×C per hour for a maximum of 20 hours to avoid damaging the batteries.
For a slow charge, or "trickle charge" process, Duracell recommends "a maintenance charge of indefinite duration at C/300 rate". Some chargers do this after the charge cycle, to offset the natural self-discharge rate of the battery. To maximize battery life, the preferred charge method of NiMH cells uses low duty cycle pulses of high current rather than continuous low current.
Safety
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 C/10 h (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. As such, cells have a vent. Hydrogen will be emitted from the vent in the event of serious overcharging.
Additional information
Voltage depression ("memory effect") from repeated partial discharge can occur, but is reversible through charge cycling.
Discharging
Under a light load (0.5 amp), the starting voltage of a freshly charged AA NiMH cell in good condition is about 1.4 volts; some measure almost 1.5 volts. Mid-discharge at a load of 1 amp, the output is about 1.2 volts; at 2 amps, about 1.15 volts; the total effective differential internal resistance is about 0.05 ohms.
Over-discharging
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 won'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.
Self-discharge
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 (> 8000mAh) 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 degrees Celsius (68F). These cells are marketed as "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 standard NiMH cells. The highest capacity low-self-discharge cells have 2000-2450mAh and 850mAh capacities for AA and AAA cells, respectively, compared to 2800mAh and 1000mAh for standard AA and AAA cells. 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.
Environmental impact
NiMH batteries are commonly considered to have lower environmental impact than NiCd batteries, due to absence of toxic cadmium. The overall environmental impact of mining the various alternate metals that form the negative electrode may be more or less than cadmium, depending on the metal, mining method, and environmental practices of the mine.
Most industrial nickel is recycled, due to the relatively easy retrieval of the metal from scrap, and due to its high value.
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 mAh, 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 mAh for C and 10000 mAh for D sizes.
PP3 NiMH batteries are available - these usually have an output voltage of 8.4 V (1.2 x 7) and a capacity of roughly 200mAh.
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. The ability to recharge hundreds of times can save money and resources.
They are often used in digital cameras and work well in this application. 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 can handle these current levels and maintain their full capacity.
Certain devices which were designed to operate using primary carbon zinc or alkaline chemistry cells will not function when one uses NiMH cells as substitutes. This is because of the lower voltage of NiMH cells. Even though the nominal NiMH voltage is lower, it sustains for the length of the discharge cycle, because the low internal resistance allows NiMH cells to deliver a near-constant voltage until they are almost completely discharged. Alkaline discharge voltage drops more towards the end of the discharge cycle.
Lithium ion batteries have a higher energy density than nickel-metal hydride batteries.
See also
Bibliography
External links
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