ICS1702 datasheet(21/28 Pages) ETC

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ICS1702

•Temperature Slope and Maximum Temperature

Temperature slope and/or maximum temperature termination may

have to be used for equipment that has high dynamic current

demands while operating from the battery during fast charge. Also,

users who do not have a well regulated constant current source

available may have to use temperature termination. In general,

utilizing temperature slope as the primary termination method with

maximum temperature termination as a safety back-up feature is

the best approach. When using temperature slope termination, the

actual current should not be appreciably lower than the selected

rate in order that termination of fast charge occurs due to the

battery reaching full charge rather than by the timer expiring.

Temperature termination methods require that the thermal sensor

be in intimate contact with the battery. A low thermal impedance

contact area is required for accurate temperature sensing. The area

and quality of the contact surface between the sensor and the

battery directly affects the accuracy of temperature sensing.

Thermally conductive adhesives may have to be considered in

some applications to ensure good thermal transfer from the battery

case to the sensor.

The thermal sensor should be placed on the largest surface of the

battery for the best accuracy. The size of the battery is also a

consideration when using temperature termination. The larger the

battery the lower the surface area to volume ratio. Because of this,

larger batteries are less capable in dissipating internal heat.

Additional considerations beyond the basics mentioned above may

be involved when using temperature slope termination where

sudden changes in ambient temperature occur or where forced air

cooling is used. For these applications, the surface area of the

thermal sensor in contact with the battery compared to the surface

area of the thermal sensor in contact with the ambient air may be

significant. For example, bead type thermistors are relatively small

devices which have far less thermal capacity compared to most

batteries. Insulating the surface of the thermistor that is in contact

with the ambient air should help minimize heat loss by the

thermistor and maintain accuracy.

•Voltage Slope Termination

The voltage slope termination method used by the ICS1702

requires a nearly constant current flow into the battery during fast

charge. Equipment that draws a known constant current while the

battery is charging may use the voltage slope termination method.

This constant current draw must be added to the fast charge

current. Using the voltage slope termination method for equipment

that randomly or periodically requires moderate current from the

battery during fast charge needs evaluation. Equipment that

randomly or periodically requires high current from the battery

during fast charge may cause a voltage inflection that results in

termination before full charge. A voltage inflection can occur due

to the charge current decreasing or fluctuating as the load changes

rather than by the battery reaching full charge. The voltage slope

method will terminate charge based on voltage inflections that are

characteristic of a fully charged battery.

Charging sources that produce decreasing current as fast charge

progresses may also cause a voltage inflection that may result in

termination before full charge. For example, if the charge current is

supplied through a resistor or if the charging source is a constant

current type that has insufficient input voltage, the current will

decrease and may cause a termination before full charge. Other

current source abnormalities that may cause a voltage inflection

that is characteristic of a fully charged battery are inadequate ripple

and noise attentuation capability or charge current decreasing due

to thermal drift. Charging sources that have any of the above

characteristics need evaluation to access their suitability for the

application if the use of the voltage slope termination is desired.

When using voltage slope termination, the controller soft start

stage, built-in noise filtering, and fast charge timer operate

optimally when the constant current source charges the battery at

the rate selected. If the actual charge current is significantly less

than the rate selected, the conditioning effect of the soft start stage

and the controller noise immunity are lessened. Also, the fast

charge timer may cause termination based on time duration rather

than by the battery reaching full charge due to inadequate charge

current.

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