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NICKEL METAL HYDRIDE BATTERIES
Overview
As electronic products have come to feature more
sophisticated functions, more compact sizes and
lighter weights, the sources of power that operate
these products have been required to deliver increasingly higher levels of energy . To meet this
requirement, nickel-metal hydride batteries have
been developed and manufactured with nickel hydroxide for the positive electrode and hydrogen-absorbing alloys, capable of absorbing and releasing hydrogen at high-density levels , for the negative electrode.
Because Ni-MH batteries have about twice the
energy density of Ni-Cd batteries and a similar
operating voltage as that of Ni-Cd batteries, they are
expected to become a mainstay in the next generation of rechargeable batteries.
Construction
Nickel-metal hydride batteries consist of a positive
plate containing nickel hydroxide as its principal active
material, a negative plate mainly composed of hydrogen-absorbing alloys, a separator made of fine fibers,
an alkaline electrolyte, a metal case and a sealing
plate provided with a self-resealing safety vent. Their
basic structure is identical to that of Ni-Cd batteries.
With cylindrical nickel-metal hydride batteries, the
positive and negativ e plates are seperated by the
separator , wound into a coil, inserted into the case ,
and sealed by the sealing plate through an electrically
insulated gasket.
With prismatic nickel-metal hydride batteries, the
positive and negative plates are sandwiched together
in layers with separators between them, inserted into
the case, and sealed by the sealing plate.
NICKEL METAL HYDRIDE HANDBOOK, PA GE
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August 2000
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NICKEL METAL HYDRIDE BATTERIES - CONTINUED
Structure of Nickel Metal Hydride Batteries
Cap (+)
Positive
Electrode
Collector
Case
( )
Insulator
Safety V ent
Cylindrical T ype
Sealing Plate
Insulation Ring
Negative Electrode
Separator
Positive Electrode
Insulation Ring
Insulator
Negative Electrode
Case
Prismatic T ype
Principle of Electrochemical Reaction Involved in Batteries
Hydrogen-absorbing Alloys
Hydrogen-absorbing alloys have a comparatively
short history which dates back about 20 years to the
discovery of NiFe, MgNi and LaNi
capable of absorbing hydrogen equivalent to about a
thousand times of their own volume, generating metal
hydrides and also of releasing the hydrogen that they
absorbed. These hydrogen-absorbing alloys combine
metal (A) whose hydrides generate heat exothermically with metal (B) whose hydrides generate heat
endothermically to produce the suitable binding
energy so that hydrogen can be absorbed and released at or around normal temperature and pressure
levels. Depending on how metals A and B are combined, the alloys are classified into the following
types: AB (TiFe, etc.), AB
etc.) and A
2
B (Mg2Ni, etc.). From the perspective of
2
(ZnMn2, etc.), AB5 (LaNi5,
charge and discharge efficiency and durability , the
field of candidate metals suited for use as electrodes
in storage batteries is now being narrowed down to
AB
5
type alloys in which rare-earth metals, especially
metals in the lanthanum group, and nickel serve as
the host metals; and to AB
2
titanium and nickel serve as the host metals.
Panasonic is now focusing its attention on AB
alloys which feature high capacity, excellent charge
and discharge efficiency, and excellent cycle life. It
has developed, and is now employing its own MmNi
alloy which uses Mm (misch metal = an alloy consisting of a mixture of rare-earth elements) for metal A.
5
alloys. They are
type alloys in which the
5
type
Principle of Electrochemical Reaction
Involved in Batteries
Nickel-metal hydride batteries employ nickel hydroxide for the positive electrode similar to Ni-Cd batteries. The hydrogen is stored in a hydrogen-absorbing
alloy for the negative electrode, and an aqueous
solution consisting mainly of potassium hydroxide for
the electrolyte. Their charge and discharge reactions
are shown below .
Positive
electrode
Negative
electrode
Overall
reaction
(
:
M
Ni(OH)
:
:
M
:M
Ni
hydrogen-absorbing alloy;
+
2
++
HO MH
2
()
+
OH
2
As can be seen by the overall reaction given above,
the chief characteristics of the principle behind a
nickel-metal hydride battery is that hydrogen moves
from the positive to negative electrode during charge
and reverse during discharge, with the electrolyte
taking no part in the reaction; which means that there
is no accompanying increase or decrease in the
electrolyte. A model of this battery’s charge and
discharge mechanism is shown in the figure on the
5
following page. These are the useful reactions taking
place at the respective boundary faces of the positive
and negative electrodes, and to assist one in understanding the principle, the figure shows how the
reactions proceed by the transfer of protons (H
Charge
-
OH
Discharge
Charge
--
e
Discharge
Charge
Discharge
Cap
Safety V ent
Sealing Electrode
Positive Electrode
Separator
NiOOH
NiOOH
H
ab
+
+
OH
ab
+
MH
:
absorbed hydrogen)
HO
2
ab
-
+
e
+
).
NICKEL METAL HYDRIDE HANDBOOK, PA GE
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August 2000
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NICKEL METAL HYDRIDE BATTERIES - CONTINUED
The hydrogen-absorbing alloy negative electrode
successfully reduces the gaseous oxygen given off
from the positive electrode during overcharge by
sufficiently increasing the capacity of the negative
electrode which is the same method employed by NiCd batteries. By keeping the battery’s internal pressure constant in this manner, it is possible to seal the
battery.
MH
x
H
M
(Negative Electrode
Hydrogen-absorbing Alloy)
H
H
Schematic Discharge of Ni-MH Battery
Charge
+
H
Discharge
+
H
H
H
(Positive Electrode
H
O
+
O
+
Nickel Hydroxide)
OH
Ni
OH
Ni
Features
••
• Similarity with Ni-Cd batteries
••
These batteries have similar discharge characteristics to those of Ni-Cd batteries.
••
• Double the energy density of conventional
••
batteries
Nickel-metal hydride batteries have approximately
double the capacity compared with Panasonic’s
standard Ni-Cd batteries.
1.8
1.6
1.4
1.2
Voltage (V)
1.0
0.8
0 200 400 600 800 1000 1200 1400 1600 1800 2000
••
• Cycle life equivalent to 500 charge and
••
Discharge Capacity (mAh)
discharge cycles
Like Ni-Cd batteries, nickel-metal hydride batteries
can be repeatedly charged and discharged for about
500 cycles. (example: IEC charge and discharge
conditions)
••
• Rapid charge in approx. 1 hour
••
Nickel-metal hydride batteries can be rapidly
charged in about an hour using a specially designed
charger.
••
• Excellent discharge characteristics
••
Since the internal resistance of nickel-metal hydride
batteries is low , continuous high-rate discharge up to
3CmA is possible, similar to Ni-Cd batteries.
Size : KR17/43
Charge : 1CmA X 1.2h
Discharge : 0.2CmA
Temperature: 20˚C
P-120AS
HHR160A
Ni-MH
HHR200A
Ni-MH
Ni-Cd
2200
2000
HHR200A
Ni-MH
1800
1600
HHR160A
Ni-MH
1400
1200
P-120AS
Ni-Cd
1000
Capacity (mAh)
800
600
400
0 1 2 3 4 5
Discharge Current (A)
NICKEL METAL HYDRIDE HANDBOOK, PA GE
Size : HR17/43
Charge: 1CmA X 1.2h
Temp.: 20˚C
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August 2000
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NICKEL METAL HYDRIDE BATTERIES - CONTINUED
Five Main Characteristics
As with Ni-Cd batteries, nickel-metal hydride batteries
have five main characteristics: charge, discharge,
storage life, cycle life and safety.
••
• Charge characteristics
••
Like Ni-Cd batteries, the charge characteristics of nickelmetal hydride batteries are affected by current, time and
temperature. The battery voltage rises when the charge
current is increased or when the temperature is low.
The charge efficiency differs depending on the current,
time, temperature and other factors.
Nickel-metal hydride batteries should be charged at a
°
temperature ranging from 0
current of 1C or less. The charge efficiency is
particularly good at a temperature of 10
Repeated charge at high or low temperatures causes
the battery performance to deteriorate. Fur thermore,
repeated overcharge should be avoided since it will
downgrade the battery performance.
Refer to the section on recommended charge methods
for details on how to charge the batteries.
• Charge characteristics
2.0
1.8
1.6
1.4
1.2
Voltage (V)
1.0
0.8
0.6
0 20 40 60 80 100 120 140 160
Charge Capacity (%) (Nominal Capacity Ratio)
C to 40°C using a constant
°
C to 30°C.
Charge : 120%
Temperature: 20C
Model : HHR160A
1C
0.33C
0.1C
• Charge temperature characteristics at various
charge rates
110
100
90
Charge
80
70
60
50
Capacity Ratio (%)
40
-10 0 10 20 30 40 50 60 70
••
• Discharge characteristics
••
0.1C x 12h
0.33C x 4h
1C x 1.2h
Discharge : 0.2C to 1.0V
Temperature: 20C
Model : HHR160A
Charge Temperature
0.1C
1C
0.33C
The discharge characteristics of nickel-metal hydride
batteries are affected by current, temperature, etc.,
and the discharge voltage characteristics are flat at
1.2V, which is almost the same as for Ni-Cd
batteries. The discharge voltage and discharge
efficiency decrease in proportion as the current rises
or the temperature drops.
repeated
high
charge and discharge of these batteries under
discharge cut-off voltage conditions (more than 1.1V
As with Ni-Cd batteries,
per cell) causes a drop in the discharge voltage
(which is sometimes accompanied by a
simultaneous drop in capacity). The discharge
characteristics can be restored by charge and
discharge to a discharge end voltage of down to 1.0V
per cell.
• Charge temperature characteristics at 1C charge
2.0
1.8
1.6
1.4
1.2
Voltage (V)
1.0
0.8
0.6
0 20 40 60 80 100 120 140 160
Charge Capacity (%) (Nominal Capacity Ratio)
Charge : 1CmA x 120%
Model : HHR160A
0C
20C
40C
• Discharge characteristics
2.0
1.8
1.6
1.4
1.2
Voltage (V)
1.0
0.8
0.6
0 20 40 60 80 100 120
Discharge Capacity (%) (Nominal Capacity Ratio)
0.2C 1C 3C
Charge : 1CmA x 1.2h
Temperature: 20C
Model : HHR160A
NICKEL METAL HYDRIDE HANDBOOK, PA GE
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August 2000
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NICKEL METAL HYDRIDE BATTERIES - CONTINUED
•
Discharge temperature characteristics at 1C discharge
2.0
1.8
1.6
1.4
1.2
Voltage (V)
1.0
0.8
0.6
0 20 40 60 80 100 120 140 160
20˚C
-10˚C
0˚C
Charge Capacity (%) (Nominal Capacity Ratio)
Charge : 1CmA x 1.2h
Temperature: 20˚C
Model : HHR160A
• Discharge temperature characteristics
120
Self-discharge is affected by the temperature at
which the batteries are left standing and the length of
time during which they are left standing. It increases
in proportion as the temperature or the shelf-standing
time increases. Panasonic’s nickel-metal hydride
batteries have excellent self-discharge
characteristics that are comparable to those of Ni-Cd
batteries.
Cycle Life Characteristics
The cycle life of these batteries is governed by the
conditions under which they are charged and discharged, temperature and other conditions of use.
Under proper conditions of use (example: IEC charge
and discharge conditions), these batteries can be
charged and discharged for more than 500 cycles.
• Cycle life characteristics
100
80
60
40
Capacity Ratio (%)
20
0
-20 -10 0 10 20 30 40 50
Discharge Temperature (˚C)
1C
3C
Charge : 1CmA x 1.2h
Temperature: 20˚C
Discharge : Cut-off Voltage 1.0V
Model : HHR160A
Storage characteristics
These characteristics include self-discharge
characteristics and restoration characteristics after
long-term storage. When batteries are left standing,
their capacity generally drops due to self-discharge,
but this is restored by charge.
• Self discharge characteristics
100
90
80
70
60
50
Capacity Ratio (%)
Charge : 1CmA x 1.2h
40
Discharge: 1CmA to 1.0V/cell
30
01234
Storage Period (weeks)
Ni-Cd (P-120AS)
Temp.: 20˚C
Ni-MH (HHR160A)
Temp.: 45˚C
120
100
80
60
40
Capacity Ration (%)
20
0 100 200 300 400 500
Number of Cycles (cycle)
Temperature : 20˚C
Model : HHR160A
Safety
When the internal pressure of these batteries rises
due to overcharge, short-circuiting, reverse charge or
other abuse or misuse, the self-resealing safety vent
is activated to prevent battery damage. Panasonic’s
nickel-metal hydride batteries have similar safety
characteristics as Panasonic Ni-Cd batteries.
NICKEL METAL HYDRIDE HANDBOOK, PA GE
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August 2000
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