Original question:
[FAQ] BQ76952: Common questions for TI's BQ769x2 Family of Battery Monitors
A cell open-wire (OW) condition can be described as a connection break between a cell in the pack and the cell attachment to the PCB of the BMS’s Analog Front End (AFE). In an AFE with no open-wire detection circuitry, the voltage at the cell input pin of the AFE device may persist for some time on the board-level capacitor, leading to incorrect voltage readings and/or delayed protective actions.
The BQ769x2 device family supports the detection of an open-wire condition, known as Cell Open Wire (COW) detection, where the device enables a small current source from each cell to VSS at programmable intervals. As seen in Figure-1.
Figure-1. Current path for OW current source. Cell voltage not affected if there is no open-wire condition.
If a cell input pin is floating due to an open-wire condition, this current source will discharge the capacitance, causing the voltage at the pin to slowly drop. Figure-2 shows how the discharge of this capacitor affects the voltages seen by the cell measurement pins of the device.
As the voltage of the capacitor drains, the open-wired cell’s voltage will continuous to decrease at the device’s inputs, this will eventually trigger a Cell Undervoltage protection fault (CUV) on that particular cell.
The drop of voltage in the capacitor of the open-wire cell input, also causes the adjacent cell above it to proportionally increase in voltage as the open-wired cell’s capacitance is drained, which eventually leads to a Cell Overvoltage protection fault (COV).
Figure-2. When OW condition is present, current source drains OW capacitor, while adjacent top cell increases in voltage measurement proportionally.
Eventually, the voltage change due to the OW condition may trigger Permanent Fail (PF) protections by the device, like the cell voltage on the OW input pins dropping low enough to trigger a Safety Undervoltage PF on the particular cell, or a Safety Overvoltage PF on the cell above it. As represented by Figure-3.
Figure-3. OW source eventually completely drains OW cell capacitor, causing adjacent top cell input to now measure twice the original cell voltage with respect to the input pins.
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.
Note: The BQ769x2 does not have a dedicated OW bit to indicate that an open-wire condition has occurred. However, one can infer this state based on the COV/CUV protections and cell voltages.
How can the Cell Open Wire detection be configured?
The Cell Open Wire current source can be configured to be enabled at periodic intervals set by the Settings:Cell Open-Wire:Check Time configuration register, with a setting of 0 disabling this check entirely. This provides programmability in the average current drawn from ≈0.65-nA to ≈165-nA, based on the typical current level of 55-µA.
For a more detailed explanation of the Cell Open Wire detection feature, read Section 5.3.24 Cell Open Wire of the Technical Reference Manual.
