ICS1702 datasheet(7/28 Pages) ETC

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ICS1702

Figure 8: Cell temperature and

thermistor voltage slope

Table 1 shows the decrease in thermistor voltage the last minute

before full charge required by the ICS1702 at various charge rates.

The thermistor voltage slope should exceed the listed value to

ensure charge termination. Note that changes in thermistor

location, cell size or large ambient temperature fluctuations can

affect the slope to some degree. Refer to the Applications

Information section and Temperature Slope and Maximum

Temperature section for more information on thermistor mounting.

Table 1: Slope vs. Charge Rate

Temperature Slope Termination

Temperature slope termination is based on the battery producing an

accelerated rate of heating as the amount of readily chargeable

material dimishes at full charge. An increase in battery (cell)

heating due to the charging reaction will occur at a much faster rate

than a change due to a warming ambient temperature. Note the

effect of 0.5

°C fluctuations in ambient temperatures resulting in

slight variations in the temperature slope as shown in Figure 7.

However, the increase in cell temperature near the end of charge

causes a much larger change in the temperature slope that can be

easily detected and used as a trigger for fast charge termination.

Figure 7: Cell temperature and

temperature slope

The rate of change in cell temperature can be determined by

measuring the change in voltage across a negative temperature

coefficient thermistor as shown in Figure 8. The resistance of an

NTC thermistor changes in proportion in the change in temperature

of the thermistor. The ICS1702 measures the decreasing resistance

as a drop in voltage and calculates the thermistor voltage slope,

shown in Figure 8. The controller terminates fast charge based on

the selected charge rate and the calculated slope.

Charge Rate

Thermistor Voltage Slope

(-V/min.)

>C/2

0.040

C/2 to C/3

0.028

<C/3

0.018

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