A nickel–metal hydride battery, abbreviated NiMH or Ni–MH, is a
type of rechargeable battery. The chemical reaction at the positive
electrode is similar to that of the nickel–cadmium cell (NiCd), with
both using nickel oxide hydroxide (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
3.1 Trickle charging
3.2 ΔV charging method
3.3 ΔT charging method
3.5 Loss of capacity
4.3 Low self-discharge
5 Compared to other battery types
6.1 Consumer electronics
6.2 Electric vehicles
7 See also
9 External links
Disassembled NiMH AA battery:
Outer metal casing (also negative terminal)
Negative electrode with current collector (metal grid, connected to
Separator (between electrodes)
See also: History of the battery
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.[clarification needed]
Development was sponsored over nearly two decades 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
Interest grew in the 1970s with the commercialisation of the
nickel–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
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. Modern NiMH
cells were based on this design. The first consumer-grade NiMH
cells became commercially available in 1989.
In 1998, Ovonic Battery Co. improved the Ti–Ni alloy structure and
composition and patented its innovations.
In 2008, more than two million hybrid cars worldwide were manufactured
with NiMH batteries.
European Union and due to its Battery Directive, nickel–metal
hydride batteries replaced Ni–Cd batteries for portable consumer
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.
BASF produced a modified microstructure that helped make NiMH
batteries more durable, in turn allowing changes to the cell design
that saved considerable weight, allowing the gravimetric energy
density to reach 140 watt-hours per kilogram.
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The negative electrode reaction occurring in a NiMH cell is
H2O + M + e− ⇌ OH− + 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− ⇌ NiO(OH) + H2O + e−
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,
or aluminium. Some cells use higher-capacity negative electrode
materials based on AB2 compounds, where A is titanium or vanadium, and
B is zirconium or nickel, modified with chromium, cobalt, iron, or
manganese. 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. Hydrophilic
polyolefin nonwovens are used for separation.
Charging voltage is in the range of 1.4–1.6 V per 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
The simplest of the safe charging methods 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
Panasonic NiMH charging manual warns that overcharging
for long enough can damage a battery and suggests limiting the total
charging time to 10–20 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
electrodes 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, where the design remains essentially the same
as in older
NiCd units, except for an increase in the trickle-charging
resistor value.
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.
ΔV charging method
NiMH charge curve
In order to prevent cell damage, fast chargers must terminate their
charge cycle before overcharging occurs. One method is to monitor the
change of voltage with time. 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 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
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,
Panasonic recommends to terminate charging when the voltage drops
5–10 mV per cell from the peak voltage. Since this method
measures the voltage across the battery, a constant-current (rather
than a constant-voltage) charging circuit is used.
ΔT charging method
The 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
Duracell suggest a maximal
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 ΔT and the ΔV
charging methods, both manufacturers recommend a further period of
trickle charging to follow the initial rapid charge.
NiMH cell that popped its cap due to failed safety valve
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 gets too high.
Modern NiMH cells contain catalysts to handle gases produced by
displaystyle ce 2H2+O2->[ text catalyst ]2H2O
). However, this only works with overcharging currents of up to
0.1 C (that is, 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 gas to form, potentially rupturing the cell. Therefore, cells
have a vent to release the gas in the event of serious
NiMH batteries are made of environmentally friendly materials. The
batteries contain only mildly toxic substances and are recyclable.
Loss of capacity
Voltage depression (often mistakenly attributed to the memory effect)
from repeated partial discharge can occur, but is reversible with a
few full discharge/charge cycles.
A fully charged cell supplies an average 1.25 V/cell during discharge,
declining 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). 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.
Complete discharge of multi-cell packs can cause 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 into reverse
polarity (i.e. positive anode/negative cathode). Some cameras, GPS
receivers and PDAs detect the safe end-of-discharge voltage of the
series cells and perform an auto-shutdown, but devices such as
flashlights and some toys do not.
Irreversible damage from polarity reversal is a particular danger,
even when a low voltage-threshold cutout is employed, when 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.
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 and longer battery life. The self-discharge
is 5–20% on the first day and stabilizes around 0.5–4% per day at
room temperature. But at 45 °C it is
approximately three times as high.
The low self-discharge nickel–metal hydride battery (LSD NiMH) has a
significantly lower rate of self-discharge. The innovation was
introduced in 2005 by Sanyo, under their
Eneloop brand. By using
an improved electrode separator and improved positive electrode,
manufacturers claim the cells retain 70–85% 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 typical NiMH chargers. These cells are
marketed as "hybrid", "ready-to-use" or "pre-charged" rechargeables.
Retention of charge depends in large part on the battery's impedance
or internal resistance (the lower the better), and on its 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 2500 mAh capacity, compared to
2700 mAh for high-capacity AA NiMH cells.
Compared to 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. Typical alkaline
AA-size batteries, which offer approximately 2600 mAh capacity at
low current demand (25 mA), provide only 1300 mAh capacity
with a 500 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.
Devices that were designed to operate using primary alkaline chemistry
(or zinc–carbon/chloride) cells may 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 nearly constant voltage
until they are almost completely discharged. Thus battery-level
indicators designed to read alkaline cells overstate the remaining
charge when used with NiMH cells, as 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. They also produce a higher voltage (3.2-3.7V nominal),
and are thus not a drop-in replacement for alkaline batteries without
circuitry to reduce voltage.
As of 2005[update], nickel–metal hydride batteries constituted three
percent of the battery market.
High-power Ni–MH battery of
Toyota NHW20 Prius, Japan
Nickel–metal hydride 24 V battery pack made by VARTA, Museum
Autovision, Altlussheim, Germany
NiMH batteries have replaced
NiCd for many roles, notably small
rechargeable batteries. NiMH batteries are commonly available in AA
(penlight-size) batteries. These have nominal charge capacities (C) of
1.1–2.8 Ah at 1.2 V, measured at the rate that discharges
the cell in 5 hours. Useful discharge capacity is a decreasing
function of the discharge rate, but up to a rate of around 1×C (full
discharge in 1 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 can
operate many devices designed for that voltage.
Main articles: Electric vehicle, Battery electric vehicle, Electric
Patent encumbrance of large automotive NiMH batteries
Applications of NiMH electric-vehicle batteries include all-electric
plug-in vehicles such as the
General Motors EV1, first-generation
Toyota RAV4 EV, Honda EV Plus,
Ford Ranger EV
Ford Ranger EV and
Hybrid vehicles such as the
Toyota Prius, Honda Insight, Ford Escape
Chevrolet Malibu Hybrid
Chevrolet Malibu Hybrid and
Honda Civic Hybrid
Honda Civic Hybrid also use them.
Stanford R. Ovshinsky
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
General Motors EV1 and Dodge Caravan EPIC
minivan. In October 2000, the patent was sold to Texaco, and a week
Texaco was acquired by Chevron. Chevron's
provides these batteries only to large OEM orders.
General Motors shut
down production of the EV1, citing lack of battery availability as a
Cobasys control of NiMH batteries created a patent
encumbrance for large automotive NiMH batteries.
Comparison of battery types
Gas diffusion electrode
List of battery sizes
List of battery types
Lithium iron phosphate battery
Patent encumbrance of large automotive NiMH batteries
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Nickel Metal Hydride Battery" by Martin G. Klein, Michael
Eskra, Robert Plivelich and Paula Ralston
Nickel Metal Hydride (NiMH) Handbook and Application Manual
Chevron/Texaco's patent on the NiMH battery
NiMH battery charging and safety
gel / VRLA
Lithium iron phosphate
Dual carbon battery