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Ni-MH Rechargeable Batteries
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
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
1.2 volts per cell. A smaller voltage depression and
capacity loss occurs if the discharge is cut off between
1.15 volts 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.
1.35
1.25
1.15
1.05
0.95
0.85
0 0.25 0.5 0.75 1.0
Voltage (V)
Time (Hours)
Effects on Ni-MH cell capacity due to repetitive partial
discharges.
[Conditions: Charge: (Cycle #1– #21) = 1C to -∆V = 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)]
Cycle #2
Cycle #18
Cycle #21
Cycle #19
Cycle #20
Cycle #1
FIGURE 5.9.1
Performance Characteristics (cont.)
5.9 Voltage Depression (“Memory Effect”)