Ni-MH Rechargeable Batteries
Ch
arging
Sealed
Nickel-Met
al
Hydride Batteries
(con
t.)
6.3.1 Duracell’s Recommendation: Three-Step Charge Procedure
For fast charging and optimum performance, Duracell recommends a three-step procedure that pro- vides a means of rapidly charging a nickel-metal hydride battery to full charge without excessive overcharging or exposure to high temperatures. The steps in sequential order are:
1)Charge at the 1C rate, terminated by using dT/dt = 1°C(1.8°F) /minute.
2)Apply a C/10 top-up charge, terminated by a timer after 1/2 hour of charge.
3)Apply a maintenance charge of indefinite
duration at C/300 rate.
The three-step charging method should be used with a backup temperature cutoff of 60°C (140°F).
6.3.2 Low-Rate Charge (≈12 hours)
Charging at a constant current at the C/10 rate with time-limited charge termination is a convenient method to fully charge nickel-metal hydride batteries. At this current level, the generation of gas will not exceed the oxygen recombination rate. The charge should be terminated after 120 percent charge input, or approximately 12 hours for a fully discharged bat-
tery. Excessive overcharging should be avoided, as it can damage the battery.
The temperature range for this charge method is 0°C to 45°C (32°F to 113°F), with optimum performance being obtained between 15°C to 30°C (59°F to 86°F).
6.3.3 Quick Charge (≈4 hours)
Nickel-metal hydride batteries can be efficiently and safely charged at higher rates than described in Section 6.3.2. Charge control is required in order to terminate the charge when the rate of oxygen recombi- nation is exceeded or the battery temperature rises excessively. A fully discharged battery can be charged at the C/3 rate terminated with a -ΔV = 10 mV/cell. In addition, a timer control set to a 120 percent charge input (3.6 hours) and a temperature cutoff of 60°C (140°F) should be used as a backup termination to
avoid exposing the battery to excessively high tempera- tures. This charging method may be used in an ambient temperature range of 10°C to 45°C (50°F to 113°F). A top-up charge is not necessary if this termination method is used.
At the C/3 rate, a dT/dt termination method should not be used because the rate of temperature increase may not be sufficient to terminate the charge.
6.3.4 Fast Charge (≈1 hour)
Another method of charging nickel-metal hydride batteries in even less time is to charge at the C/2 to 1C constant current rates. At these high charge rates, it is essential that the charge be terminated early during overcharge. However, timer control is inade- quate, as the time needed for charge can not be pre- dicted — a partially charged battery could easily be overcharged while a fully discharged one could be undercharged, depending on how the timer control
is set.
With fast charging, the decrease in voltage (-ΔV) and the increase in temperature (ΔT) can be used to terminate the charge. For better results, termination of fast charge can be controlled by sensing the rate of temperature increase (dT/dt). A temperature increase of 1°C (1.8°F) per minute with a backup temperature cutoff of 60°C (140°F) is recommended. A top-up charge of C/10 for 30 minutes should follow to ensure a full charge.
