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Ni-MH Rechargeable Batteries
5.7 Internal Impedance
DURACELL nickel-metal hydride batteries have low
internal impedance because they are manufactured using
cells designed with thin plate electrodes which offer large
surface areas and good conductivity. Figure 5.7.1 shows
the change in internal impedance with depth of discharge.
As demonstrated, the impedance remains relatively constant
during most of the discharge. Towards the end of the dis-
charge, the impedance increases due to the conversion of
the active materials to a non-conductive form.
5.8 Self-Discharge and Charge Retention
The state-of-charge and capacity of the nickel-
metal hydride battery decrease during storage due to
self-discharge of the cells. Self-discharge results from
the reaction of residual hydrogen in the battery with the
positive electrode, as well as the slow and reversible
decomposition of the positive electrode. The rate of
self-discharge is dependent upon the length of time and
temperature at which the battery is stored — the high-
er the temperature, the greater the rate of self-dis-
charge. As illustrated in Figure 5.8.1, cells stored at
0°C (32°F) retain more of their capacity than those
stored at 20°C and 45°C (68°F and 113°F), particularly
after 30 days.
Generally, long term storage of a nickel-metal
hydride battery in either a charged or discharged condi-
tion has no permanent effect on capacity. Capacity loss
due to self-discharge is reversible and nickel-metal
hydride batteries can recover to full capacity by proper
recharging. For example, full capacity of a nickel-metal
hydride battery that was stored at room temperature
for up to one year can be restored by cycling through
repeated charge/discharge cycles.
As with operation at elevated temperatures,
however, long term storage at high temperatures can
lead to deterioration of seals and separators and should
be avoided. The recommended temperature range for
long term storage of nickel-metal hydride batteries is
10°C to 30°C (50°F to 86°F).
9
Performance Characteristics (cont.)
Internal Impedance (m
Ω
)
FIGURE 5.7.1
165
180
170
175
DR30
0 20 40 60 80 100
Discharge Capacity (%)
Internal impedance of DURACELL DR30 Ni-MH
batteries at various discharge capacities.
[Conditions: Charge: C/5 for 7.5 hours; Discharge: C/5;
Temperature: 21°C (70°F); Measurements at 1000 Hz]
Residual Capacity (%)
0 5 10 15 20 25 30
FIGURE 5.8.1
Storage Time (Days)
100
80
60
40
20
0
45°C (113°F)
0°C (32°F)
20°C (68°F)
Self-discharge characteristic of Ni-MH cells at
various temperatures.
[Conditions: Charge: C/3 for 5 hours; Discharge: C/5 to 1.0V;
Temperature: 21°C (70°F)]