The BQ76905 device supports detecting a disconnection between a cell in the pack and the cell attachment to the PCB containing BQ76905. Without this check, the voltage at the cell input pin of the BQ76905 device can persist for some time on the board-level capacitor, leading to incorrect voltage readings. The Cell Open Wire detection in the BQ76905 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 discharges the capacitance, causing the voltage at the pin to slowly drop. This drop in voltage can eventually trigger a Cell Undervoltage protection fault on that particular cell, as well as a Cell Overvoltage protection fault on the cell above it. It is important that the cell undervoltage and overvoltage protections be enabled with appropriate threshold settings for the open wire condition to be detected and the desired reaction initiated.

In NORMAL mode, the cell open wire current is enabled, one cell at a time, one cell approximately every ADSCAN, then disabled for COWNTIME FULLSCAN intervals. The parameter COWNTIME is set by the Settings:Protection:Cell Open-Wire NORMAL Check Time configuration register, and a setting of 0 disables this check entirely. The current source at each cell is only enabled during the Shared Slot measurement (which is programmable in width from 366 μs to 2.93 ms), except that it skips a Shared Slot being used to measure the stack voltage, to avoid corrupting the stack voltage measurement. Only one current source is enabled in one ADSCAN, so in a 5s system it requires 6 ADSCAN intervals to have the current source at each cell enabled (depending on the skipped slots), then the current sources are all disabled for COWNTIME FULLSCAN intervals, before the loop repeats.

This provides programmability in the average current drawn from ≈2 nA to ≈3 μA in NORMAL mode, based on the typical current source value of 55 µA. During SLEEP mode, the cell open wire current source is enabled immediately following the last burst measurement slot in immediate sequence for each cell in selected burst measurements, for the duration of a Shared Slot per cell. The selection of burst measurements is based on the Settings:Protection:Cell Open-Wire SLEEP Check Time[COWSTIME2:0]. The cell open-wire currents can be enabled or disabled in SLEEP mode using the Settings:Protection:Cell Open-Wire SLEEP Check Time[COWSEN] configuration bit.

The turn-on and turn-off of the cell open-wire current source creates a transient at the cell voltage input, and depending on the RC time constant of the input components used at the VC input pins, the transient can affect the accuracy of the Cell 1 measurement in NORMAL mode, which occurs immediately after the Shared Slot when the current source is enabled. In order to overcome this issue, the BQ76905 includes a programmable delay which is inserted between the end of each Shared Slot and the beginning of the cell measurements, to allow sufficient settling for this transient. This delay can be set from 0 to 64 ms by the Settings:Protection:Cell Open-Wire SLEEP Check Time[COWDLY] configuration parameter, which has a default of 1 ms.