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7
7
Cycle and Battery Life
7.1 Cycle Life
The cycle life of nickel-metal hydride batteries
depends on the many conditions to which the battery
has been exposed, as is true for all types of recharge-
able batteries. These include such variables as:
•
Temperature during charge and discharge
•
Charge and discharge current
•
Depth of discharge
•
Method of charge control
•
Exposure to overcharging and overdischarging
•
Storage conditions
Typically, under a C/5 charge/discharge at
normal ambient temperatures (20°C or 68°F), up to
500 cycles can be achieved with the battery delivering
at least 80 percent of its rated capacity. The gradual
decrease in capacity results from an increase in the bat-
tery’s internal resistance, caused by minor irreversible
changes in the structure of the electrodes, electrolyte
distribution and separator dry-out.
For optimum battery life and maximum cycle
life, nickel-metal hydride batteries should be operated at
or near room temperature (20°C or 68°F). Repeated
operation at extreme temperatures during charge and
discharge will adversely affect the performance of the
cells (and thus the battery), as shown graphically in
Figure 7.1.1. Operation at high temperatures, particu-
larly in the overcharged condition, can cause the cell to
vent, releasing gas and possibly electrolyte through the
safety vent. High temperatures will also hasten the
deterioration of the separator and other materials in the
cell. At temperatures below 0°C (32°F), the oxygen
recombination reaction slows down and the cell is more
sensitive to overcharging, thus gas pressure will build up
more rapidly.
Ni-MH Rechargeable Batteries
32 50 68 86 104 122
Impact on cycle life from repeated charging and
discharging at various ambient temperatures.
[Conditions: Charge: C/4 for 3.2 hours; Discharge: C/4 for 2.4 hours;
Capacity measured every 50 cycles @ 21°C (70°F): Charge: C/3 for
5 hours; Discharge: 1C to 1.0V]
100
80
60
40
20
10
Temperature (
°
F)
Cycle Life (%)
FIGURE 7.1.1
Temperature (
°
C)
0 10 20 30 40 50
90
70
50
30
17