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NICKEL-METAL HYDRIDE BATTERIES
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.
9 volt NiMH batteries usually have an output voltage of 8.4 V (1.2 × 7)
and a capacity of roughly 200mAh. Also available are eight-cell 9 volt
batteries with a nominal output voltage of 9.6 V (1.2 × 8). These are
the batteries used in our SG-5500.
The ability to recharge hundreds of times can save money and resources.
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 mAh to 2900 mAh 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).
The pioneering work on NiMH batteries – essentially based on sintered
Ti2Ni+TiNi+x alloys for the negative electrode and NiOOH-electrodes for
the positives – was performed at the Battelle Geneva Research Center
starting after its invention in 1967: The development work was sponsored
over nearly 2 decades by Daimler-Benz Comp. /Stuttgart and by Volkswagen
AG., within the framework of Deutsche Automobilgesellschaft. The
batteries showed high energy density up to 50 Wh/kg, power density up to
1000 W/kg and a reasonable deep cycle life of 500 cycles (DOD=100%).
Patent applications were filed in European countries (priority:
Switzerland), USA and Japan and the patents transferred to Daimler-Benz
Company/Stuttgart. Ref: Elektrode zur Speicherung und Aktivierung von
Wasserstoff", Inventor: K.D. Beccu, Battelle-Geneva, Swiss Priority
Application No. 6333/67-Bb3/CH/2 - (2.05.1967), Patent: DE 2317505 C2
(18.10.73). Negative electrode of Ti-Ni alloy hydride phases, US patent
US 3,669,745 (06/13, 1972), inventor: K.D. Beccu, Ph.D, Battelle-Geneva
R&D Center.
The NiMH cells developed by Philips Laboratories and French CNRS (see
above) were based on rare earth metal alloys. They suffered from the
instability of the alloys in alkaline electrolyte and consequently
insufficient cycle life.
Ovonic Battery Company in Michigan/USA altered and improved the Ti-Ni
alloy structure and composition according to their patent and licensed NiMH batteries to over 50 companies worldwide. The "invented NiMH
variation" of Ovonics consisted in special alloys with disordered alloy
structure and specific multicomponent alloy compositions. Unfortunately
linked to their composition, calendar and cycle life of such alloys
always remain very low, and all NiMH batteries manufactured at present
time in the world consist in AB5 type rare earth metal alloys.
Currently, more than 2 million hybrid cars worldwide are running with
NiMH batteries, {ref: Avicenne Conference, Nice 2008, M.A. Fetcenko/ECD} e.g.
PRIUS, LEXUS (Toyota), CIVIC, INSIGHT (Honda), FUSION (Ford) and others.
A large proportion of these batteries is manufactured by PEVE
(Panasonic) and Sanyo.
High power Ni-MH Battery of Toyota NHW20 Prius, Japan
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 aluminum. 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.
For separation hydrophilic polyolefin nonwovens are used.
The charging voltage is in the range of 1.4–1.6 V/cell. A fully-charged
cell measures 1.4–1.45 V (unloaded), and supplies a nominal average 1.25
V/cell during discharge; down to about 1.0–1.1 V/cell (further discharge
may cause permanent damage). This voltage varies depending on the
discharge rate of the cell (lower discharge loads result in an increased
voltage output for longer periods, approaching the 1.4 V unloaded cell
voltage). 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 NiCd charger should
not be used as an automatic substitute for a NiMH charger.
According to Panasonic and other NiMH cell manufacturers, the "ΔV”
method is one of the preferred charging methods for charging. The
charger measures the rate of change (signified by the symbol Δ) of the
voltage of the cell (signified by the letter V). This is illustrated in
the "NiMH Charge curve" figure. The cell or battery is rapidly charged
at a constant current of 1 C/h, where C is the capacity of the battery
(the capacity is expressed in ampere hours, or more commonly milliampere
hours (mAh). 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 "ΔV" 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.
The Δ temperature method is similar in principle to the ΔV 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.
Charging with Switched-mode power supply 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).
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
0.1C/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 0.033C/h and 0.05C/h 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 0.0033C/h". 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.
The SG-5500 AC/DC combi sensor (photoelectric and heat)
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 0.1C (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.
Voltage depression ("memory effect") from repeated partial discharge can
occur, but is reversible through charge cycling.
Under a light load (0.5 amps), the starting voltage of a freshly charged
AA NiMH cell in good condition is about 1.4 volts; some measure almost
1.5 volts. This voltage falls rapidly to about 1.25 volts at 10% Depth
of Discharge (DOD) and then remains almost constant until the cell is
over 80% discharged. The voltage then falls rapidly from about 1.2 volts
down to 0.8-1.0 volts at which the cell is considered "flat" in most
devices. 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. Nickel metal hydride batteries
provide a relatively constant voltage for most of the discharge cycle,
unlike a standard alkaline where the voltage falls steadily during
discharge.
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.
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 (>8000 mAh) are reported to have the
highest self-discharge rates.
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 70–85% of their
capacity after one year when stored at 20 °C (68 °F). These cells are
marketed as "Hybrid", "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 some standard NiMH
cells due to the larger area of the separator. The highest capacity
low-self-discharge cells have 2000–2450 mAh and 850 mAh capacities for
AA and AAA cells respectively, compared to 2800 mAh and 1000 mAh for
standard AA and AAA cells. But C types are typically higher than their
usual NiMH cousins with 4000 mAh and the D type being 8000 mAh.
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.
We at L.I.F.E. Support
Technologies® intend to include a rechargeable battery with all of our
base stations to power our Evacuation System detectors in case of power
failures making the
SG-5500’s true AC/DC units. Due to the problems with
Lithium batteries, we have opted to use NiMH batteries.
The first consumer grade NiMH cells for smaller applications appeared on
the market in 1989 as a variation of the 1970s' Nickel hydrogen battery.
Positive electrode development was done by Dr. Masahiko Oshitani from GS
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.
Nickel-metal hydride battery made by Varta, Museum Autovision,
Altlußheim, Germany
Applications of NiMH electric vehicle batteries include 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, Chevrolet Malibu 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.
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,
NiMH Charge curve
NiMH battery manual charging
There is an inherent risk with NiMH chemistry that overcharging will
cause a buildup of Hydrogen, causing the cell to rupture. Therefore,
cells have a vent. Hydrogen will be emitted from the vent in the event
of serious overcharging.
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.
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