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LW1DSE > TECH 15.01.18 18:11l 331 Lines 16997 Bytes #999 (0) @ WW
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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
oxyhydroxide (NiOOH). However, the negative electrodes use a hydrogen-absor_
bing 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 battery.
History:
ฤฤฤฤฤฤฤ
Work on NiMH batteries began at the Battelle-Geneva Research Center
following the technology's invention in 1967. It was based on sintered
Ti Ni + TiNi + x alloys and NiOOH-electrodes. Development was sponsored over
2
nearly two decades by Daimler-Benz and by Volkswagen AG within Deutsche Auto_
mobilgesellschaft, now a subsidiary of Daimler AG. The batteries' specific
energy reached 50 Wh/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 Daimler-Benz.
Interest grew in the 1970's 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 in_
sufficient 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.
Ovonic Battery Co. in Michigan altered and improved the Ti-Ni alloy
structure and composition according to their patent and licensed NiMH batte_
ries to over 50 companies. Ovonic's NiMH variation consisted of special alloys
with disordered alloy structure and specific multicomponent alloy compositions.
Unfortunately, due to their composition, the calendar and cycle life of such
alloys remains low.
A high-energy pasted electrode developed by Dr. Masahiko Oshitani from
GS Yuasa Company led to the NiMH cell.
In 2008, more than two million hybrid cars worldwide were manufactured
with NiMH batteries.
In the European Union and due to its Battery Directive, nickel-metal
hydride batteries replaced Ni-Cd batteries for portable consumer use.
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.
In 2015 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.
Electrochemistry.
ฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤ
The negative electrode reaction occurring in a NiMH cell is:
H O + M + e- <=> OH- + MH
2
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) + OH- <=> NiO(OH) + H O + e-
2 2
The metal M in the negative electrode of a NiMH cell is an intermetal_
lic compound. Many different compounds have been developed for this applica_
tion, 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-capa_
city 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 elec_
trode passes through the separator and recombines at the surface of the nega_
tive. 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 hy_
drogen ions or protons. The hydrogen ions are stored in a metal hydride struc_
ture that is the electrode. For separation hydrophilic polyolefin nonwovens
are used.
Charge.
ฤฤฤฤฤฤ
Charging voltage is in the range of 1.4-1.6 V/cell. In general, a
constant-voltage charging method can't 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. A NiCd charger isn't a
substitute for an automatic NiMH charger.
Trickle charging.
ฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤ
The simplest, safe charging method 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 or C/10 (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 sug_
gests limiting the total charging time to 10 to 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 anode 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.
V charging method.
ฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤ
In order prevent cell damage, fast chargers must terminate their char_
ge 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
unreliable.
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 char_
ge, 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 me_
thod measures the voltage across the battery, a constant current (rather than
a constant voltage) charging circuit is used.
T temperature charging method.
ฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤ
The temperature change method is similar in principle to theV method.
Because the charging voltage is nearly constant, constant-current charging de_
livers energy at a near-constant rate. When the cell isn't 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 1๘C per minute. Using a temperature sensor allows an
absolute temperature cutoff, which Duracell suggests at 60๘C. With both the
T and theV charging methods, both manufacturers recommend a further period
of trickle charging to follow the initial rapid charge.
Safety.
ฤฤฤฤฤฤ
A resettable fuse in series with the cell, particularly of the bime_
tallic 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
over-charging (2H + O - catalyst => 2 H O). However, this only works with
2 2 2
overcharging currents of up to 0.1 C (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 invol_
ves 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 hy_
drogen buildup, potentially rupturing the cell. Therefore, cells have a vent
to release the gas in the event of serious overcharging.
Nickel metal hydride batteries are made of environmentally friendly
materials. The batteries contain only mild toxins and are recyclable.
Loss of capacity.
ฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤ
Memory effect from repeated partial discharge can occur, but is rever_
sible through charge cycling.
Discharge.
ฤฤฤฤฤฤฤฤฤ
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 dama_
ge 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.
Over-discharge.
ฤฤฤฤฤฤฤฤฤฤฤฤฤฤ
Complete discharge can reverse polarity in one or more cells, which
can permanently damage them. This situation can occur in the common arrange_
ment of four AA cells in series in a digital camera, where one completely dis_
charges before the others due to small differences in capacity among the cells
When this happens, the good cells start to drive the discharged cell in rever_
se. Some cameras, GPS receivers and PDAs detect the safe end-of-discharge
voltage of the series cells and auto-shutdown, but devices such as flash_
lights and some toys don't. A single cell driving a load or a cell connected
in parallel to other cells can't suffer from polarity reversal, because no
other cells are present.
Irreversible damage from polarity reversal is a particular danger,
even when a low voltage threshold cutout is employed, should 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.
Self-discharge.
ฤฤฤฤฤฤฤฤฤฤฤฤฤฤ
NiMH cells historically had a somewhat higher self-discharge rate
(equivalent to internal leakage) than NiCd cells. This self-discharge rate
varies greatly with temperature, where lower storage temperature leads to
slower discharge rate 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 45๘C it is approximately three times as high.
Low self-discharge.
ฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤ
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 se_
parator and improved positive electrode, manufacturers claim the cells retain
70% to 85% of their capacity when stored one year at 20๘C, compared to about
half for normal NiMH batteries. They are otherwise similar to other NiMH
batteries, and can be charged in the typical chargers. These cells are marke_
ted 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 bat_
tery 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-dis_
charge rate. Some batteries may have overcome this tradeoff using thin sepa_
rators with more precise manufacturing and by using a more advanced sulfona_
ted 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-dis_
charge AA cells have 2-2.6Ah capacity, and AAA 1Ah, compared to 2.8Ah and 1.3
Ah for high-capacity AA and AAA NiMH cells.
Compared to other battery types.
ฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤฤ
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, lar_
gely due to their lower internal resistance. Alkaline batteries, which offer
approximately 3Ah capacity at low current demand (0.2A), provide only 700 mAh
capacity with a 1A load. Digital cameras with LCDs and flashlights can draw
over 1A, quickly depleting them. NiMH cells can deliver these current levels
without similar loss of capacity.
Certain devices that were designed to operate using primary alkaline
chemistry (or zinc-carbon/chloride) cells wont function with NiMH cells.
However most devices compensate for the voltage drop of an alkaline battery
as it discharges down to about one volt. Low internal resistance allows NiMH
cells to deliver a near-constant voltage until they are almost completely
discharged. Battery level indicators overstate the remaining charge if it was
designed to read alkaline cells. The voltage of alkaline cells decreases ste_
adily 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.
As of 2005 nickel metal hydride batteries constituted three percent
of the battery market.
Applications.
ฤฤฤฤฤฤฤฤฤฤฤฤ
NiMH batteries have replaced NiCd for many roles, notably small re_
chargeable batteries. NiMH batteries are commonly used for AA (penlight-size)
batteries. These have nominal charge capacities (C) of 1.1-2.8Ah 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 doesn't 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 will operate many devices
designed for that voltage.
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