In the previous post we talked about the charging and discharging behavior or a Li-ion battery. On our way to a full fledged fuel gauge we need to first measure battery voltage. Figure 1 shows the battery voltage measurement circuit using and ADC, in this case a MCP3421 18-bit ADC (U1).
Figure 1 Battery Voltage Measurement
Since the MCP3421 device has an internal reference voltage, the measurable maximum input voltage range is limited to the internal voltage reference voltage of up to 2.048V. To measure the input voltage higher than the internal reference, a voltage divider is used, which is formed by R1, R2, and R3. The R3 is optional and is used to calibrate the R1 and R2 component tolerance. By choosing the series resistance value of the voltage divider to be very high (> 1 MΩ), the current losses due to the voltage divider is negligible.
In the example circuit as shown in Figure 1, the ADC is configured as single ended by connecting the positive input pin (VIN+) to the battery voltage, while the negative input pin (VIN-) to the VSS. The ADC output is available to the MCU via the I2C bus line.
Figure 2 shows the discharge curve of a 3.7V Li-Polymer battery (3.7V, 170 mAH). The curve shows that the battery voltage reduces linearly until it reaches about 80% of its full capacity.
Figure 2 Li-Polymer Battery Voltage Discharging Curve
Since the battery discharging characteristics are very linear until the point where the curve falls off sharply, measuring only the battery voltage is an alternative low-cost method to estimate the current status of the battery. In this case, the measured battery voltage can be compared with the fuel values in the lookup table in the MCU firmware.
The circuit shown can be used for measuring the battery voltage of any battery type. When the circuit is used, the voltage divider (R1, R2, R3) must be properly adjusted in order to keep the maximum input voltage (or the voltage at VIN+ pin when the battery is fully charged) to the ADC device is less than the ADC internal reference voltage (2.048V).
Although using the voltage alone is not sufficient to represent the battery fuel status, this method is widely used for simple and cost-sensitive applications because of its straightforward implementation.
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