Duracell Ni-MH Charging Sealed Nickel-Metal Hydride Batt eries, Techniques for Charge Control

Ni-MH Rechargeable Batteries

ChargingSealedNickel-MetalHydrideBatteries(cont.)

Duracell recommends the charge termination method described in Section 6.3.1.

The voltage of the nickel-metal hydride battery during charge depends on a number of conditions, including charge current and temperature. Figures

6.1.3and 6.1.4 show the voltage profile of the nickel- metal hydride battery at different ambient temperatures and charge rates, respectively. The battery voltage rises with an increase in charge current due to an increase in the “IR” drop and overpotential during the electrode reaction. The battery voltage decreases with increasing temperature as the internal resistance and overpotential during the electrode reaction decrease.

A rise in temperature and pressure at high charge rates occurs and underscores the need for prop- er charge control and effective charge termination when “fast charging.” Excessive pressure and tempera- ture increases can result in activation of cell vents or battery safety electronics, as described in Section 6.4.

Temperature also affects charge efficiency. Charge efficiency decreases at higher temperatures due to the increasing evolution of oxygen at the positive electrode. Thus, charging at high temperatures results in lower capacity. At lower temperatures, charge effi- ciency is high due to decreasing oxygen evolution. However, oxygen recombination is slower at lower tem- peratures and a rise in internal cell pressure may occur depending on the charge rate.

Proper charging is critical not only to obtain maximum capacity on subsequent discharges but also to avoid high internal temperatures, excessive over- charge and other conditions which could adversely affect battery life.

6.2 Techniques for Charge Control

The characteristics of the nickel-metal hydride battery define the need for proper charge control in order to terminate the charge and prevent overcharging or exposure to high temperatures. Each charge control technique has its advantages and disadvantages. For example, higher capacity levels are achieved with a 150 percent charge input, but at the expense of cycle life; long cycle life is attained with a 105 to 110 percent charge input, albeit with slightly lower capacity due to

FIGURE 6.1.3

	10.0
	9.5
				0°C	(32°F)
(V)	9.0
				21°C (70°F)
Voltage
	8.5
	8.0
	7.5
	0	0.2	0.4	0.6		0.8	1.0	1.2

Charge Time (Hours)

Charge voltage of DURACELL DR30 Ni-MH batteries at various temperatures.

[Conditions: Discharge: C/5 to 6.0V @ 21°C (70°F); Charge: 1C to -ΔV = 60mV]

	FIGURE 6.1.4
	10.0
	9.5
					C (2.4A)
(V)	9.0
					C/5 (0.48A)
	8.5
Voltage
	8.0
	7.5
	7.0
	0	1.0	2.0	3.0		4.0

Charge Capacity (Ah)

Charge voltage of DURACELL DR30 Ni-MH batteries at various rates.

[Conditions: Discharge: C/5 to 6.0V; Charge: 1C to -ΔV = 60mV, C/5 to

7.5 hrs.; Temperature: 21°C (70°F)]

less charge input. Thermal cutoff charge control may reduce cycle life because higher temperatures are reached during the charge; however, it is useful as a backup control in the event that the primary termina- tion method is not effective during charge.

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