SLUUBY1 User guide

SLUUBY1B December 2020 – April 2022 BQ76942

4.1.2 Usage of VC Pins for Cells Versus Interconnect

If the BQ76942 device is used in a system with fewer than 10-series cells, the additional cell inputs can be utilized to improve measurement performance. For example, a long connection may exist between two cells in a pack, such that there may be significant interconnect resistance between the cells, such as shown in Figure 4-1 between CELL-A and CELL-B. By connecting VC7 close to the positive terminal of CELL-B, and connecting VC8 close to the negative terminal of CELL-A, more accurate cell voltage measurements are obtained for CELL-A and CELL-B, since the I·R voltage across the interconnect resistance between the cells is not included in either cell voltage measurement. Since the device reports the voltage across the interconnect resistance and the synchronized current in DASTATUS1–3(), the resistance of the interconnect between CELL-A and CELL-B can also be calculated and monitored during operation. It is recommended to include the series resistance and bypass capacitor on cell inputs connected in this manner, as shown below.

Note: It is important that the differential input for each cell input not fall below –0.3 V (the Absolute Maximum data sheet limit), with the recommended minimum voltage of –0.2 V. Therefore, it is important that the I·R voltage drop across the interconnect resistance does not cause a violation of this requirement.

Figure 4-1 Using Cell Input Pins for Interconnect Measurement

If this connection across an interconnect is not needed (or it is preferred to avoid the extra resistor and capacitor), then the unused cell input pins should be shorted to adjacent cell input pins, as shown in Figure 4-2 for VC8.

Figure 4-2 Terminating an Unused Cell Input Pin

The Settings:Configuration:Vcell Mode configuration register is used to specify which cell inputs are used for actual cells. The device uses this information to disable cell voltage protections associated with inputs that are used to measure interconnect or are not used at all. Voltage measurements for all inputs are reported in a 16-bit format (in units of mV) as well as a 32-bit format (in units of raw ADC counts), irrespective of whether they are used for cells or not.