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6.2.5 Delta Temperature Cutoff (∆TCO)
6.2.6 Rate of Temperature Increase (dT/dt)
6.3 Charging Methods
Ni-MH Rechargeable Batteries
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 ∆TCO charge termination. A top-up charge is not
necessary with this termination method.
14
Charging Sealed Nickel-Metal Hydride Batteries (cont.)
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 -∆V in terminating a fast charge.
The dT/dt method senses the start of the overcharge
earlier than the -∆V 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.
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.
FIGURE 6.2.2
3.0
2.5
2.0
1.5
1.0
0 100 200 300 400 500
Discharge Capacity (Ah)
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)]
- ∆V= 60mV
dT/dt = 1°C(1.8°F)/min
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.
6.2.4 Temperature Cutoff (cont.)