Duracell Ni-MH manual Performance Cha racteristics, Voltage Depression Memory Effect

Ni-MH Rechargeable Batteries

PerformanceCharacteristics(cont.)

5.9 Voltage Depression (“Memory Effect”)

Although many years of premium performance can be enjoyed from a nickel-metal hydride battery that is properly handled, the capacity delivered in each charge/discharge cycle will eventually begin to decrease. This inevitable decrease in capacity can be accelerated by overcharging, storage or usage at high temperatures, or through poor matching of cells within a pack. Often, battery users who experience short service life have incorrectly attributed capacity loss to a phenomenon called “memory effect.”

The term memory effect is used synony- mously with the term “voltage depression.” Voltage depression is a scientifically measurable characteristic of all batteries, however, nickel-cadmium batteries demon- strate particularly acute sensitivity. A properly designed application with nickel-metal hydride batteries will result in neither permanent performance loss nor perceivable temporary capacity decreases from this characteristic.

A reversible drop in voltage and loss of capacity may occur when a nickel-metal hydride battery is par- tially discharged and recharged repetitively without the benefit of a full discharge, as illustrated in Figure 5.9.1. After an initial full discharge (Cycle #1) and charge, the cell is partially discharged to 1.15 volts and recharged for a number of cycles. During this cycling, the dis- charge voltage and capacity drop gradually in very small increments (Cycles #2 to #18). On a subsequent full discharge (Cycle #19), the discharge voltage is depressed compared to the original full discharge (Cycle #1).

Because the cell appears to “remember” the lower capacity, this voltage depression phenomenon is often referred to as memory effect. However, the cell can be quickly restored to full capacity with a few full discharge/charge cycles, as indicated in Cycles #20 and #21.

The voltage drop occurs because only a portion of the active materials in the cell is discharged and recharged during shallow or partial discharging. The active materials that have not been cycled change in

FIGURE 5.9.1

	1.35
	1.25
(V)	1.15
Voltage						Cycle #2
	1.05
	0.95						Cycle #1
				Cycle #18			Cycle #21
						Cycle #20
	0.85				Cycle #19
	0	0.25	0.5	0.75	1.0

Time (Hours)

Effects on Ni-MH cell capacity due to repetitive partial discharges.

[Conditions: Charge: (Cycle #1– #21) = 1C to -ΔV = 12mV. Discharge: Cycle #1 = 1C to 1.0 V, (Cycle #2 – #18) = 1C to 1.15V, (Cycle #19 – #21) = 1C to 1.0V; Temperature: 21°C (70°F)]

physical characteristics and increase in resistance. Subsequent full discharge/charge cycling will restore the active materials to their original state.

The extent of voltage depression and capacity loss depends on depth of discharge and can be avoided by discharging the battery to an appropriate cutoff voltage. Voltage depression is most apparent when the discharge is terminated at higher cutoff voltages, such as

1.2volts per cell. A smaller voltage depression and capacity loss occurs if the discharge is cut off between

1.15volts to 1.10 volts per cell. Discharging to 1.0 volts per cell should not result in significant voltage depression or capacity loss during subsequent discharges.

A device properly designed with nickel-metal hydride batteries will minimize the effects of voltage depression and capacity loss. The voltage depression and capacity loss in DURACELL nickel-metal hydride batteries is only a small fraction (less than 5 percent in worst cases) of the battery’s capacity and most users

will never experience a perceptible performance loss.

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