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Ni-MH Rechargeable Batteries
5.1 General Characteristics
The discharge characteristics of the nickel-metal
hydride cell are very similar to those of the nickel-
cadmium cell. The charged open circuit voltage of both
systems ranges from 1.25 to 1.35 volts per cell. On
discharge, the nominal voltage is 1.2 volts per cell and
the typical end voltage is 1.0 volt per cell.
Figure 5.1.1 illustrates the discharge character-
istics of nickel-metal hydride and nickel-cadmium
rechargeable cells of the same size. As shown, the volt-
age profile of both types of cells is flat throughout most
of the discharge. The midpoint voltage can range from
1.25 to 1.1 volts per cell, depending on the discharge
load. Figure 5.1.1 can also be used to compare the
capacity of the two rechargeable types. Note that the
capacity of the nickel-metal hydride cell is typically up to
40 percent higher than that of a nickel-cadmium cell of
equivalent size.
5.2 Discharge Characteristics: Effect of
Discharge Rate and Temperature
Typical discharge curves for DURACELL nickel-
metal hydride batteries under constant current loads at
various temperatures are shown in Figures 5.2.1 to
5.2.3. Discharge voltage is dependent on discharge
current and discharge temperature.
FIGURE 5.1.1
1.5
1.4
1.3
1.2
1.1
1.0
.9
Voltage (V)
Ampere-Hour Capacity (%)
Comparison of discharge voltage and capacity of
same-size Ni-MH and Ni-Cd cells.
[Conditions: Charge: C/3 for 5 hours, Temperature: 21°C (70°F)]
0 20 40 60 80 100 120 140 160
Ni-MH
C/5
C/5
Ni-Cd
5
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Performance Characteristics
FIGURE 5.2.1
Voltage (V)
Discharge Capacity (Ah)
FIGURE 5.2.2
Discharge Capacity (Ah)
Voltage (V)
C (2.4A)
Voltage (V)
Discharge Capacity (Ah)
8.5
8.0
7.5
7.0
6.5
6.0
5.5
0 0.5 1.0 1.5 2.0 2.5
C/5 (0.48A)
C (2.4A)
C (2.4A)
FIGURE 5.2.3
Voltage and capacity of DURACELL DR30 Ni-MH
batteries at various discharge temperatures
and rates.
[Conditions: Charge: 1C to -∆V = 60mV @ 21°C (70°F)]
Temperature: -20°C (-4°F)
8.5
8.0
7.5
7.0
6.5
6.0
5.5
0 0.5 1.0 1.5 2.0 2.5
C/5 (0.48A)
C (2.4A)
Temperature: 45°C (113°F)
8.5
8.0
7.5
7.0
6.5
6.0
5.5
C/5 (0.48A)
C (2.4A)
0 0.5 1.0 1.5 2.0 2.5
Temperature: 21°C (70°F)
C/5 (0.48A)
Temperature: 0°C (32°F)
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