5 Accumulated Charge Integration of Coulomb Counter

BQ769x2 coulomb counter offers multiple current values for readout over the serial communications interface, including two using separate hardware digital filters, CC1 and CC2, as well as a firmware filter CC3.

To calculate the SOC, the CC1 filter can be used to generate a 16-bit current measurement , with one output generated every 250ms when the device is operating in NORMAL mode. CC1 current data is available from the 0x0075 DASTATUS5() subcommand.

The integrated passed charge is available as a 64-bit value from the 0x0076 DASTATUS6() subcommand, which includes the upper 32-bits of accumulated charge in units of userAh, the lower 32-bits of accumulated charge as the fractional portion, and a 32-bit accumulated time over which the charge has been integrated in units of seconds. The accumulated charge integration and timer can be reset using the 0x0082 RESET_PASSQ() subcommand. If the device undergoes a partial reset or is reset using the RST_SHUT pin, the 0x0082 RESET_PASSQ() needs to be sent to make sure the charge accumulation is properly initialized.

The accumulated charge is equal to the direct sum of the integer and fractional portions. For example, 0xFFFFFFFD is the integer portion of the measurement, which in 2s complement format, the data represents -3 in decimal. And, 0x7FFFFFFF is the readout fractional portion, which is always a positive number between zero and 2³² , the data represents 0.5 in decimal. The sum is measuring -2.5 mAh of accumulated charge.

The LSB size of coulomb counter is 7.6uV but the current less than LSB/R_sense can also be detected. The Coulomb counter was designed so that even if the system has a lower current, the internal circuitry still retains quantization from one sample to another. BQ769x2 is capable of sensing currents that give a sense resistor voltage far below 7.6uV and accumulating the charge.

Figure 5-1 gives the small discharge currents measurements test results across the 0.3mohm sense resistor. The currents below LSB/R_sense - 25mA can also be accumulated. BQ769x2 has 3-4 uV offset voltage under room temperature so that positive accumulated charge can be observed when applying zero diacharging current. The accumulated charge becomes zero when the discharging current becomes around 1.5mA.

Meanwhile, BQ769x2 can also support synchronized voltage and current measurement for SOC calculation. The 0x0071 DASTATUS1(), 0x0072 DASTATUS2(), 0x0073 DASTATUS3(), and 0x0074 DASTATUS4() subcommands provide raw ADC counts in 32-bit format of the cell voltage measurements, as well as the synchronized current measurements taken simultaneously with each cell voltage measurement. The data is generated in 24-bit format by the data converter but provided in 32-bit format. The 24-bit data is contained in the lower 3 bytes of the 32-bit data, and is sign-extended to create the upper byte. With the synchronized voltage and current measurement, user can filter and process the data with custom filtering algorithm, without the quantization to 16-bit when using CC1 current.