AN417

Principle of the inflexion method

Doc ID 2074 Rev 2

9/21

3

Principle of the inflexion method

Every 4 seconds, the analog to digital converter (ADC) of the ST6210 microcontroller

measures the battery voltage and temperature. If the temperature is above a predetermined

threshold (40 degrees Celsius for example), fast charge is terminated.

The battery voltage is actually measured several times in series, and an average of the

measurements is made, to reduce measurement errors due to high frequency noise (for

example generated by a switched mode charging current).

Further, a rolling average of the present and previous measurements is made, to remove

low frequency noise due to electrochemical battery voltage variations (see previous

description of this technique in References [1.]).

This averaged battery voltage data is used to extract a time derivative, by calculating the

difference between present voltage and the voltage n samples earlier. Because of the

voltage digitization, which is measured by the ADC of the MCU, the first derivative curve

features a typical discontinuous aspect (see Figure 5).

For this reason, the MCU calculates a digital smoothing of this discontinuous derivative. The

inflexion point is then identified by detecting the first highest summit encountered on the

smoothed derivative curve (point B in Figure 5).

When this summit is detected, fast charge is terminated.

Figure 5.

Inflexion point method

battery voltage

voltage derivative

smoothed derivative

charging time ->

B

battery voltage and derivative