The BQ76922 device supports detecting a disconnection between a cell in the pack and the cell attachment to the PCB containing BQ76922. Without this check, the voltage at the cell input pin of the BQ76922 device may persist for some time on the board-level capacitor, leading to incorrect voltage readings. The Cell Open Wire detection in the BQ76922 device operates by enabling a small current source from each cell to VSS at programmable intervals. If a cell input pin is floating due to an open wire condition, this current will discharge the capacitance, causing the voltage at the pin to slowly drop. This drop in voltage will eventually trigger a Cell Undervoltage protection fault on that particular cell, as well as a Cell Overvoltage protection fault on the cell above it. Eventually, the voltage will drop low enough to trigger a Safety Undervoltage PF on the particular cell, or a Safety Overvoltage PF on the cell above it. It is important that the undervoltage and overvoltage protections and PFs be enabled with appropriate threshold settings for the open wire condition to be detected and the desired reaction initiated.

The cell open wire current will be enabled at a periodic interval set by the Settings:Cell Open-Wire:Check Time configuration register, with a setting of 0 disabling this check entirely. The current source is enabled once every interval for a duration of the ADC measurement time (which is 3ms by default). This provides programmability in the average current drawn from ≈0.65nA to ≈165nA, based on the typical current level of 55µA. After each Check Time interval completes, during the next measurement loop the cell open wire current source is enabled on the cell above the cell being measured (with the current source enabled on the lowest cell while the topmost cell is being measured). This is scheduled to avoid the cell open wire current corrupting the cell voltage measurements. During SLEEP mode, the cell open wire current source is enabled in similar fashion during the next set of measurements that occur after the timer has expired.