Timed Charge, Voltage Plateau Zero ΔV

Ni-MH Rechargeable Batteries

ChargingSealedNickel-MetalHydrideBatteries(cont.)

The following summary explains some of the recommended methods for charge control. The charac- teristics of each of these methods are illustrated in Figure 6.2.1. In many cases, several methods are employed, particularly for high rate charging.

6.2.1 Timed Charge

Under the timed charge control method, the charge is terminated after the battery is charged for a predetermined length of time. This method should be used only for charging at low rates (less than C/3) to avoid excessive overcharge because the state-of-charge of the battery, prior to charging, cannot always be determined. If a timed charge termination is used, a time of 120 percent charge input is recommended with a backup temperature cutoff of 60°C (140°F).

Voltage drop is widely used with nickel-cadmi- um batteries. With this technique, the voltage during charge is monitored and the charge is terminated when the voltage begins to decrease. This approach can be used with nickel-metal hydride batteries, but as noted in Section 6.1, the voltage drop of the nickel- metal hydride battery is not as prominent as that of the nickel-cadmium battery and may be absent in charge currents below the C/3 rate, particularly at elevated temperatures. The voltage sensing circuitry

6.2. 3 Voltage Plateau (Zero ΔV)

Since the nickel-metal hydride battery does not always show an adequate voltage drop, an alter- nate method used is to terminate the charge when the voltage peaks and the slope is zero, rather than waiting for the voltage to drop. The risk of over-

FIGURE 6.2.1

-Δ

Voltage (V)

DifferentialTemperature )tT/(dd

(V)Voltage

Temperature(T)

TCO

Temperature (T)

dT/dt

Temperature Differential Output (dT/dt)

Charge Time (t)

Charge characteristics of Ni-MH batteries using various charge termination methods.

must be sensitive enough to terminate the charge when the voltage drops, but not so sensitive that it will terminate prematurely due to noise or other normal voltage fluctuations. A charge rate of 1C and a 5 to 10 millivolt per cell drop is recommended for the nickel-metal hydride battery with a backup tem- perature cutoff of 60°C (140°F). A top-up charge is not necessary with this charge termination method.

charge is reduced as compared to the -ΔV method.

If this method is employed, a charge rate of 1C and a backup temperature cutoff of 60°C (140°F) is recom- mended. A top-up charge can follow to ensure a full charge. Duracell does not recommend this termina- tion method because of the risk of premature cutoff.

6.2. 4 Temperature Cutoff

Another technique for charge control is to monitor the temperature rise of the battery and ter- minate the charge when the battery has reached a temperature which indicates the beginning of over- charge. It is difficult, however, to precisely determine

this point because it is influenced by ambient tempera- ture, cell and battery design, charge rate, and other factors. A cold battery, for instance, may be over- charged before reaching the cutoff temperature, while a warm battery may be undercharged.