OW detection when negative cell voltage

Question

Hello, 
 My customer wants to measure cell voltages in the range of -1.2V to +3.5V . I assume there might be limitations in using the S-ADC in this case. 
 In particular, they are very interested in understanding how the open wire detection function operates. Since they are unable to use the S-ADC , they are considering detecting open wires based on the measured cell voltages in the main controller. 
 Question 1: If an open wire condition occurs, what voltage will be observed between the affected cell terminals? Question 2: For the unused S-ADC pins, is it acceptable to leave them open? 
 Thank you for your support.

mvrablic · Accepted Answer

The open-wire detection on the S pins uses a resistor between the SxP and SxN connections to look for a change in cell voltage. So without a voltage to move some small current across that resistor it wouldn't be detectable near 0V even if the S-ADC could measure below 0V. With battery cells typically a range check is sufficient for avoiding this near 0V case. 
 Are they measuring fuel cells for this application or is it still battery cells? If batteries, why the negative voltage requirement? If it's a busbar in the stack you can use the adjacent 2 cell S-ADC connections to check the busbar wire connections, all while leaving the SxP and SxN of the busbar channel open.)

kevintom98 · Answer

To add to this, here is the OW circuit marked in case C15 disconnects (RED cross), when the OW switch(circled in BLUE) is closed, it introduces a 18kOhm+2kOhm resistor across the cell which will translate as change in cell voltage.

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OW detection when negative cell voltage