6
Ni-MH Rechargeable Batteries
6.1 General Principles
Recharging is the process of replacing energy
that has been discharged from the battery. The subse-
quent performance of the battery, as well as its overall
life, is dependent on effective charging. The main crite-
ria for effective charging are:
•
Choosing the appropriate rate
•
Limiting the temperature
•
Selecting the appropriate termination technique
The recharging characteristics of nickel-metal
hydride batteries are generally similar to those of
nickel-cadmium batteries. There are some distinct
differences, however, particularly on the requirements
for charge control because the nickel-metal hydride
battery is more sensitive to overcharging. Caution
should be exercised before using a nickel-cadmium
battery charger interchangeably for both battery types
because it may not optimally charge a nickel-metal
hydride battery, particularly on high rate chargers.
The most common charging method for the
nickel-metal hydride battery is a constant current
charge with the current limited in order to avoid an
excessive rise in temperature. Limiting the charge
current also reduces the risk of exceeding the rate of
the oxygen recombination reaction to prevent cell
venting.
Figure 6.1.1 compares the voltage profiles of
nickel-metal hydride and nickel-cadmium batteries dur-
ing charge at a constant current rate. The voltages of
both systems rise as the batteries accept the charge. As
the batteries approach 75 to 80 percent charge, the
voltages of both battery types rise more sharply due to
the generation of oxygen at the positive electrode.
However, as the batteries go into overcharge, the volt-
age profile of the nickel-metal hydride battery does not
exhibit as prominent a voltage drop as the nickel-
cadmium battery.
In Figure 6.1.2, the temperature profiles of the
nickel-metal hydride and nickel-cadmium batteries are
compared during charge at a constant current charge
rate. Throughout the first 80 percent of charge, the
temperature of the nickel-cadmium battery rises gradu-
ally because its charge reaction is endothermic (absorbs
heat). The temperature of the nickel-metal hydride
battery, on the other hand, rises quickly because its
charge reaction is exothermic (releases heat). After
80 to 85 percent of charge, the temperature of both
battery types also rises due to the exothermic oxygen
recombination reaction, causing the voltage to drop as
the batteries reach full charge and go into overcharge.
Both the voltage drop after peaking (-∆V) and
the temperature rise are used as methods to terminate
the charge. Thus, while similar charge techniques can
be used for nickel-metal hydride and nickel-cadmium
batteries, the conditions to terminate the charge may
differ because of the varying behavior of the two bat-
tery systems during charge. To properly terminate
charging of DURACELL nickel-metal hydride batteries,
55
50
45
40
35
30
25
20
15
59
68
77
86
95
104
113
122
131
0 20 40 60 80 100 120
Charge Input (% of Typical Capacity)
FIGURE 6.1.2
Temperature (
°
C)
Temperature (
°
F)
Ni-MH
Typical charge temperature characteristics of Ni-MH
and Ni-Cd batteries.
[Conditions: Charge: 1C @ 21°C (70°F) to -∆V = 10mV/cell]
Ni-Cd
Ni-MH
2.0
1.8
1.6
1.4
1.2
1.0
Charge Input (% of Typical Capacity)
FIGURE 6.1.1
Voltage /Cell (V)
Typical charge voltage characteristics of Ni-MH and
Ni-Cd batteries.
[Conditions: Charge: 1C @ 21°C (70°F) to -∆V = 10mV/cell]
0 20 40 60 80 100 120
Ni-Cd
Ni-MH
11
Charging Sealed Nickel-Metal Hydride Batteries
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