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.
It is common to refer to most NiMH products as Batteries, even though the word Battery refers to the grouping of multiple cells. As a result sizes AA, AAA, C and D are technically Cells while the 9V size is a real battery.
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, 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.
The negative electrode reaction occurring in a NiMH battery is
On the positive electrode, nickel oxyhydroxide (NiOOH) is formed,
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.
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.
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).
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.
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.
Under a light load (.5 Amp), the starting voltage of a freshly charged AA NiMH battery 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 .05 ohms.
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.
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 (> 8000mAh) are reported to have the highest self-discharge rates.
Low self discharge batteries have lower 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.
Most industrial nickel is recycled, due to the relatively easy retrieval of the metal from scrap, and due to its high value.
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 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 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 after not many 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.
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' insistent "Big Picture" focus on large companies and purchases has not been popular with some small companies and lone individuals who want to buy directly from them.
NiMH batteries were discovered before Ovonics. The patents in question are not for all NiMH batteries but specific methods and types.