August 2000
NICKEL METAL HYDRIDE HANDBOOK, PAGE
10
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 nickel-
metal 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
°
C to 40
°
C using a constant
current of 1C or less. The charge efficiency is
particularly good at a temperature of 10
°
C to 30
°
C.
Repeated charge at high or low temperatures causes
the battery performance to deteriorate. Furthermore,
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
•Charge temperature characteristics at 1C charge
•Charge temperature characteristics at various
charge rates
••
••
• Discharge characteristics
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. As with Ni-Cd batteries,
repeated charge and discharge of these batteries under
highdischarge cut-off voltage conditions (more than 1.1V
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.
•Discharge characteristics
Charge Capacity (%) (Nominal Capacity Ratio)
Voltage (V)
1.6
1.4
1.2
1.0
0.8
0 20 40 60 80 100 120 140 160
Charge : 120%
Temperature: 20C
Model : HHR160A
1.8
0.6
2.0
1C
0.33C
0.1C
Charge Capacity (%) (Nominal Capacity Ratio)
Voltage (V)
1.6
1.4
1.2
1.0
0.8
0 20 40 60 80 100 120 140 160
Charge : 1CmA x 120%
Model : HHR160A
1.8
0.6
2.0
0C
20C
40C
110
100
80
60
40
-10 0 10 20 30 40 50 60 70
Charge Temperature
Capacity Ratio (%)
90
70
50
0.1C x 12h
0.33C x 4h
1C x 1.2h
Charge
0.33C
0.1C
1C
Discharge : 0.2C to 1.0V
Temperature: 20C
Model : HHR160A
Discharge Capacity (%) (Nominal Capacity Ratio)
Voltage (V)
1.6
1.4
1.2
1.0
0.8
0 20 40 60 80 100 120
Charge : 1CmA x 1.2h
Temperature: 20C
Model : HHR160A
1.8
0.6
2.0
0.2C 1C 3C