Duracell Ni-MH manual Charging Methods, Delta Temperature Cutoff Δtco

Ni-MH Rechargeable Batteries

ChargingSealed Nickel-MetalHydrideBatteries (cont.)

6.2.4 Temperature Cutoff (cont.)

Usually this method is used in conjunction with other charge control techniques primarily to terminate the charge in the event that the battery reaches exces- sive temperatures before the other charge controls

activate. A charge rate of 1C and a temperature cutoff at 60°C (140°F) is recommended. A top-up charge is not recommended if this termination method is used.

6.2.5 Delta Temperature Cutoff (ΔTCO)

This technique measures the battery tempera- ture rise above the starting temperature during charging and terminates the charge when this rise exceeds a pre- determined value. In this way, the influence of ambient temperature is minimized. The cutoff value is depen- dent on several factors, including cell size, configuration and number of cells in the battery, and the heat capacity of the battery. Therefore, the cutoff value should be

determined for each type of battery. This value will be greater for nickel-metal hydride batteries than for nickel-cadmium batteries. A charge rate of 1C and a temperature change of 15°C (27°F) with a backup temperature cutoff of 60°C (140°F) is recommended for ΔTCO charge termination. A top-up charge is not necessary with this termination method.

6.2.6 Rate of Temperature Increase (dT/dt)

In this method, the change in temperature with time is monitored and the charge is terminated when a predetermined rate of temperature rise is reached. The influence of ambient temperature is reduced. A dT/dt cut- off is a preferred charge control method for nickel-metal hydride batteries because it provides long cycle life. Figure 6.2.2 shows the advantage of using a dT/dt method compared to -ΔV in terminating a fast charge. The dT/dt method senses the start of the overcharge earlier than the -ΔV method. The battery is exposed to less overcharge and overheating, resulting in less loss of cycle life. A charge rate of 1C and a temperature increase of 1°C (1.8°F) per minute with a back-up temperature cut- off of 60°C (140°F) is recommended for dT/dt. A top-up charge of C/10 for 1/2 hour is also recommended.

Discharge Capacity (Ah)

FIGURE 6.2.2

3.0

2.5

-ΔV= 60mV

2.0dT/dt = 1°C(1.8°F)/min

1.5

1.0

0	100	200	300	400	500
		Cycle Number

Cycle life and capacity of DURACELL DR30 Ni-MH batteries as a function of charge termination.

[Conditions: Charge: 1C; Discharge: C/5 to 6.0V; Cycled to 70% of

initial capacity; Temperature: 21°C (70°F)]

6.3 Charging Methods

Nickel-metal hydride batteries can be charged employing the same methods used for charging nickel-cad- mium batteries. However, the charge termination tech- nique may differ because of the varying behavior of the two battery systems. For proper charging of nickel-metal hydride batteries, the charge termination technique used should be appropriate for the particular charge rate. The charge rate and appropriate termination technique is sum- marized in Table 6.3.1.

Some of the various methods used to properly charge nickel-metal hydride batteries are explained in

Sections 6.3.1 to 6.3.5. In order to optimize performance, Duracell recommends a three-step charge procedure.

Charge Rate	Termination Technique
1C to C/2	Voltage or temperature based
C/2 to C/3	Voltage based
C/3 to C/10	Not recommended
C/10 and below	Time limited

Table 6.3.1 Recommended charge termination techniques for particular charge rates.

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