6.3.5 Trickle Charge
6.4 Thermal Devices
16
Ni-MH Rechargeable Batteries
A number of applications require the use of
batteries which are maintained in a fully-charged state.
This is accomplished by trickle charging at a rate that
will replace the loss in capacity due to self-discharge.
In these applications, a
trickle charge
at a C/300 rate is
recommended. The preferred temperature range for
trickle charging is between 10
°
C to 35
°
C (50
°
F to
95
°
F). Trickle charge may be used following any of the
previously discussed charging methods.
Charging Sealed Nickel-Metal Hydride Batteries (cont.)
DURACELL nickel-metal hydride batteries con-
tain a temperature sensing device and thermal protec-
tive devices. Thermal protective devices terminate
charge/discharge in the event high temperatures are
reached. This protection is particularly important when
fast charging methods are used. The types of devices
used are:
1)
Negative Temperature Coefficient (NTC)
Thermistor:
This device senses internal bat-
tery temperature and provides this informa-
tion by means of a calibrated resistance
value to an external control circuit. The
thermistor
is attractive because the control
can be set, external to the battery, to meet
the particular conditions of the charge. This
device is used in dT/dt charge control.
2)
Thermostat:
This bimetal thermal protec-
tive device operates at a fixed temperature
and is used to cut off the charge (or dis-
charge) when a pre-established internal bat-
tery temperature or current is reached.
These temperature cutoff (TCO) devices
reset automatically after the overtempera-
ture or overcurrent condition has decreased
below a reset threshold.
3)
Thermal Fuse:
This device is wired in series
with the cell stack and will open the circuit
when a predetermined temperature is
reached. Thermal fuses are included as a
protection against thermal runaway and are
normally set to open at approximately 91
°
C
(196
°
F). This device cannot be reset once
opened.
4)
Positive Temperature Coefficient (PTC)
Device:
This is a resettable device whose
resistance rapidly increases at a predeter-
mined current, thereby reducing the current
in the battery to a low and acceptable level.
The
PTC
device will respond to high current
beyond design limits (e.g. a short circuit) and
acts like a fuse. Unlike a one-time fuse, the
PTC device will reset to its low resistance
state when the latching current is removed.
It will also respond to high temperatures
around the PTC device, in which case it
operates like a temperature cut-off (TCO)
device.
The location of thermal devices in the battery
assembly is critical to ensure that they will respond prop-
erly as the temperature may not be uniform throughout
the battery. Thermal devices in DURACELL nickel-metal
hydride batteries are set so the cells are not exposed to
temperatures above 91
°
C (196
°
F). The inclusion of
thermal protective devices in DURACELL nickel-metal
hydride batteries helps ensure safe battery operation.
