Duracell Ni-MH manual Charging Sealed Nickel-Metal Hydride Batteries, General Principles

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Ni-MH Rechargeable Batteries

ChargingSealedNickel-Metal HydrideBatteries

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

Voltage /Cell (V)

FIGURE 6.1.1

2.0

Ni-Cd

1.8

1.6

Ni-MH

1.4

1.2

1.0

0 20 40 60 80 100 120

•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

Charge Input (% of Typical Capacity)

Typical charge voltage characteristics of Ni-MH and

Ni-Cd batteries.

[Conditions: Charge: 1C @ 21°C (70°F) to -ΔV = 10mV/cell]

	FIGURE 6.1.2							°
							Temperature ( F)
	55								131
	50								122
	45								113
(°C)						Ni-MH
	40								104
Temperature	35								95
							Ni-Cd
	30								86
	25								77
	20								68
	15								59
	0	20	40	60	80	100		120

Charge Input (% of Typical Capacity)

Typical charge temperature characteristics of Ni-MH and Ni-Cd batteries.

[Conditions: Charge: 1C @ 21°C (70°F) to -ΔV = 10mV/cell]

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 (-ΔV) 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,

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