Duracell Ni-MH manual Cycle and Battery Life, Cycle Life

Ni-MH Rechargeable Batteries

7Cycle and BatteryLife

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.

	FIGURE 7.1.1		Temperature ( °F)
	32	50	68	86	104	122
	100
	90
	80
Life (%)	70
	60
Cycle
	50
	40
	30
	20
	10	10	20	30	40	50
	0
			Temperature (°C)

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]

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