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Figure 5.4.1 compares the gravimetric and
volumetric energy density of nickel-metal hydride
and nickel-cadmium cells. As indicated, nickel-metal
hydride cells deliver more energy per weight or
volume than nickel-cadmium cells.
5.5 Constant Power Discharge Characteristics
The output energy characteristic of nickel-metal
hydride batteries under the constant power mode at
different power levels is shown in Figure 5.5.1.
As illustrated, the energy delivered does not vary
significantly with increasing power. The power levels
are shown on the basis of E-Rate. The E-Rate is
calculated in a manner similar to calculating the C-Rate.
For example, at the E/10 power level, the power
for a battery rated at 17.3 watt-hours is 1.73 watts.
5.6 Polarity Reversal During Overdischarge
When cells are connected in series, the cell
with the lowest capacity will reach a lower point of
discharge than the others. The more cells that are
connected in series, the greater the possibility of a cell
being fully discharged and driven into overdischarge
and polarity reversal. During reversal, hydrogen gas
evolves from the positive electrode. Hydrogen gas
will be reabsorbed by the negative electrode and
eventually oxygen gas will evolve from the negative
electrode. Extended overdischarge will lead to elevated
cell pressure and opening of the safety vent within
the nickel-metal hydride cells.
To minimize the occurrence of polarity reversal,
the cells in DURACELL rechargeable batteries have
capacities that are “matched” to each other. Device
designers can help prevent overdischarge by designing a
cutoff voltage for device operation of 1.0 volt per cell.
Ni-MH Rechargeable Batteries
8
FIGURE 5.4.1
0
50
100
150
200
Ni-Cd Ni-MH
Wh/kg
Gravimetric and volumetric energy density of Ni-Cd
and Ni-MH cells.
Wh/kg
Wh/L
Wh/L
0
5
10
20
E/10 (1.7)
E (17.3)E/3 (5.8)
FIGURE 5.5.1
Energy (Wh)
Typical energy of DURACELL DR30 batteries under
constant power discharges.
[Conditions: Charge: 1C to -∆V = 60mV; Discharge to 6.0V;
Temperature: 21°C (70°F)]
Power (W)
15
Performance Characteristics (cont.)