6

ICS1700A

Maximum Temperature Termination

Maximum temperature can be sensed using either a NTC

thermistoror a thermal switch. Maximum temperature termination

can also be bypassed if desired, although it is strongly

recommended that some form of temperature termination be used.

If an NTC thermistor is used, an internal voltage threshold

Determines when the battery is too hot to charge. As temperature

increases, the voltage across the thermistor will drop. This voltage

is continually compared to the internal voltage thresh-old. If the

thermistor voltage drops below the internal thresh-old, the OTN

indicator is activated and the controller shuts down. The controller

must be reset once the hot battery fault condition has cleared to

restart the charge sequence.

If a thermal switch is used, a 45

°C open circuit switch is

recommended. When the thermal switch opens, an internal pull-up

at the THERM pin results in a logic high which shuts down the

controller and activates the OTN indicator. The controller must be

reset once the hot battery fault condition has cleared to restart the

charge sequence.

Maximum temperature termination can be disabled by grounding

the THERM pin. See the section on Temperature Sensing for more

information.

Fast Charge Timer Termination

The controller uses a timer to limit the fast charge duration. These

times are pre-programmed, and are automatically adjusted in time

duration according to the charge rate selected. Fast charge timer

termination is best suited as a safety backup feature to limit the

duration of the fast charge stage. The fast charge timer is always

enabled and cannot be disabled. See Table 2 for more information.

Charge Termination Methods

Several charge termination schemes, including voltage slope,

maximum temperature and a fast charge timer are available. The

voltage slope method may be used with or without the maximum

temperature method. Maximum temperature and the fast charge

timer are available as backup methods.

Voltage Slope Termination

The most distinctive point on the voltage curve of a charging

battery in response to a constant current is the voltage peak that

occurs as the cell approaches full charge. By mathematically

calculating the first derivative of the voltage, a second curve can be

generated showing the change in voltage with respect to time as

shown in Figure 6. The slope will reach a maximum just before the

actual peak in the cell voltage. Using the voltage slope data, the

ICS1700A calculates the point of full charge and accurately

terminates the applied current as the battery reaches that point. The

actual termination point depends on the charging characteristics of

the particular battery.

Cells that are not thoroughly conditioned or possess an unusual cell

construction may not have a normal voltage profile. The

ICS1700A uses an alternate method of charge termination based

on a slight decrease in the voltage slope to stop charge to cells

whose voltage profile is very shallow. This method looks for a

flattening of the voltage slope which may indicate a shallow peak

in the voltage profile. The zero slope point occurs slightly beyond

the peak voltage and is shown on the voltage curve graph.

Figure 6: Voltage and slope curves showing inflection and zero slope points