Duracell Ni-MH manual Thermal Devices, Trickle Charge

Ni-MH Rechargeable Batteries

ChargingSealedNickel-Metal Hydride Batteries(cont.)

6.3.5 Trickle Charge

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.

6.4 Thermal Devices

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.

16