Nickel-metal hydride battery

A nickel-metal hydride battery, abbreviated NiMH, is a type of rechargeable battery similar to a nickel-cadmium (NiCd) battery but using a hydrogen-absorbing alloy for the negative electrode instead of cadmium. As in NiCd batteries, the positive electrode is nickel oxyhydroxide (NiOOH). A NiMH battery can have two to three times the capacity of an equivalent size NiCd. However, compared to the lithium-ion battery, the volumetric energy density is lower and self-discharge is higher.

Common AA batteries (penlight-size) have nominal charge capacities (C) ranging from 1100 mA·h to 2700 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 batteries began to appear at the end of the 1980s. 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 battery.

Applications
Applications of NiMH type batteries includes all-electric plug-in vehicles such as the Toyota RAV4 EV, General Motors EV1, Honda EV Plus, Ford Ranger EV and Vectrix scooter. Hybrid vehicles such as the Toyota Prius, Honda Insight, 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 battery is
 * $$\mathrm{H_2O + M + e^- \leftrightharpoons OH^- + MH}.$$

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,
 * $$\mathrm{Ni(OH)_2 + OH^- \leftrightharpoons NiO(OH) + H_2O + e^-.}$$

The "metal" M in the negative electrode of a NiMH battery 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 batteries 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 batteries to remain sealed in normal operation and to be maintenance-free.

NiMH batteries 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 batteries with a smart battery charger to avoid overcharging, which can damage batteries and cause dangerous conditions. A Ni-Cad charger should not be used as an automatic substitute for a NiMH charger.

The delta-V charging method
One of the preferred charging methods is the "delta-V" method, according to Panasonic and other battery manufacturers. This is illustrated in the "NiMH Charge curve" figure. The battery is rapidly charged at constant-current, at a high rate of 1C (where C is the capacity of the battery in Amp-hours and 1 is the multiplier in the equation used for calculating charging current: current(Amps) = multiplier(C) x capacity(Amp-hours)). After the battery 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 "delta-V" type battery charger will sense this drop in voltage, and when a set threshold is exceeded, the charge cycle must end, and the charge current must be stopped. 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 battery for example, which can in theory be more easily charged at a suitably chosen constant-voltage. 

The delta-temperature charging method
The delta-temperature method is similar in principle to the delta-V method. Since the charging voltage is nearly constant, if constant current charging is used, then a near constant power is entering the battery. When the battery is charging, most of this power will be converted to chemical energy. However, when the battery 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 battery 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 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. This may be what happens in some types of cheap cordless phone base stations and the cheapest battery chargers. Although this may be safe when the current is low enough, it decreases the battery capacity and longevity. 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&times;C per hour and 0.05&times;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 batteries (and most types of batteries), 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 battery, particularly of the bimetallic strip type. This fuse will open if either the current or the temperature goes too high.

Modern NiMH batteries 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.

Additional information
Voltage depression ("memory effect") from repeated partial discharge can occur, but is reversible through charge cycling.

Discharging
NiMH cells do not handle over-discharging very well. 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 shut themselves down, 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.

Self-discharge
NiMH historically had a somewhat higher self-discharge rate (equivalent to internal leakage) than NiCd in the past. However, this is no longer the case. 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 (> 4600mAh) are reported to have the highest self-discharge rates.

Low Self Discharge Batteries
A new type of nickel-metal hydride battery was introduced in 2005 that reduces self-discharge and, therefore, lengthens shelf life. By using a new separator, manufacturers claim the batteries 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.

Some brands that are currently available on the market (Nov 2007) are Accupower Acculoop, Ansmann MaxE range, Camlink Ready 2 Go, Duracell Pre-charged, Gold Peak ReCyko, Kodak Pre Charged, Nexcell EnergyOn, Panasonic R2, Rayovac Hybrid, GE/Sanyo Eneloop, Sony CycleEnergy, Titanium Power Enduro, Uniross Hybrio, Vapextech Instant and VARTA Ready2use. As there are only three manufacturers of this new type of cell (Sanyo, Panasonic, Yuasa-Delta) most of these brands are rebranded OEMs.

Low self discharge batteries appear only to be available in AA and AAA sizes and have less capacity than standard NiMH batteries. The highest capacity low self discharge batteries have 2000-2100mAh and 850mAh capacities for AA and AAA batteries, respectively, compared to 2800mAh and 1000mAh for standard AA and AAA batteries. 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.

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 batteries 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 batteries are somewhat available, but are often just an 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.

NiMH batteries are not expensive, and the voltage and performance is similar to standard alkaline batteries in those sizes; they can be substituted for most purposes. The ability to recharge hundreds of times can save a lot of 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 rechargeables.

NiMH batteries 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 less than 1000 mA·h capacity with a 1000 mA load. Digital cameras with LCDs and flashlights can draw over 1000 mA, quickly depleting alkaline batteries after a few shots. NiMH can handle these current levels and maintain their full capacity.

Sometimes, voltage-sensitive devices won't perform well because the voltage of NiMH batteries is lower than fresh disposable batteries at equivalent sizes, particularly at light loads. Even though the nominal NiMH voltage is lower, it sustains for the length of the discharge cycle, since 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.

Patent encumbrance of NiMH batteries
In 1994, General Motors acquired a controlling interest in Ovonics's battery development and manufacturing. In 2001, Texaco purchased GM's share in GM Ovonics. A few months later, Chevron acquired Texaco. In 2003, Texaco Ovonics Battery Systems was restructured into Cobasys, a 50/50 joint venture between Chevron and Energy Conversion Devices (ECD) Ovonics. Chevron's influence over Cobasys extends beyond a strict 50/50 joint venture. Chevron holds a 19.99% interest in ECD Ovonics. In addition, Chevron maintains the right to seize all of Cobasys' intellectual property rights in the event that ECD Ovonics does not fulfill its contractual obligations. On September 10, 2007, Chevron filed a legal claim that ECD Ovonics has not fulfilled its obligations. ECD Ovonics disputes this claim. Since that time, the arbitration hearing was repeatedly suspended while the parties negotiate with an unknown prospective buyer. No agreement has been reached with the potential buyer. Cobasys's patents relating to NIMH batteries expire in 2015. In her book, Plug-in Hybrids: The Cars that Will Recharge America, published in February 2007, Sherry Boschert argues that large-format NiMH batteries are commercially viable but that Cobasys refuses to sell or license them to small companies or individuals. Boschert reveals that Cobasys accepts only very large orders for these batteries. When Boschert conducted her research, major auto makers showed little interest in large orders for large-format NiMH batteries. However, Toyota employees complained about the difficulty in getting smaller orders of large format NiMH batteries to service the existing 825 RAV-4EVs. Since no other companies were willing to make large orders, Cobasys was not manufacturing nor licensing any large format NiMH battery technology for automotive purposes. Boschert concludes that "it's possible that Cobasys (Chevron) is squelching all access to large NiMH batteries through its control of patent licenses in order to remove a competitor to gasoline. Or it's possible that Cobasys simply wants the market for itself and is waiting for a major automaker to start producing plug-in hybrids or electric vehicles."

However, recently-signed Cobasys contracts demonstrate that the company is willing to use its NiMH technology in the automotive industry, specifically for use with hybrid electric vehicles. In December 2006, Cobasys and General Motors announced that they had signed a contract under which Cobasys provides NiMH batteries for the Saturn Aura hybrid sedan. In March 2007, GM announced that it would use Cobasys NiMH batteries in the 2008 Chevrolet Malibu hybrid as well. Cobasys remains unwilling to sell NiMH batteries in smaller quantities to individuals or companies interested in building or retrofitting their own PHEVs.

Saft offers their NHE NiMH Modules with capacities of 100Ah and 200Ah.

Tianjin peace Gulf Power Group Co. offers NiMH in their HP-280QNF line, with ratings of 40Ah, 80Ah, and 100Ah.