BQ76940: broken circuit detection

Hi Tony,

The BQ76940 does not have a built in open wire detection system.  With an open wire leakages will typically pull the pin voltage down.  One cell voltage will drop while the adjacent cell voltage will increase.  The ADC will limit at a voltage so the higher voltage cell may stop increasing while the lower one continues to fall to 0.  Normally the BQ76940 will get an undervoltage or overvoltage condition.  However the board may have other leakages or the connection may not be completely open which may hold a voltage which appears to be in range.  The cell balance feature of the BQ76940 may be used to pull on the cell inputs.  With an open or high impedance connection the cell would be pulled to a low voltage and be slow to return to normal.  An algorithm could step through the cells looking for unexpected voltages.  We do not have an example algorithm.

A few items to note:

If the connection to cell 5 breaks as marked in the figure above, VC5 will typically fall to VC4 and show a low voltage. In a BQ76930 or BQ76940 design there is a diode to the supply pin which may have enough leakage to reduce that drop somewhat. It may be more common for the wire from the cell to the board to break affecting both the VC5 input and the power connection.  This may have more noticeable effects with the BQ76920.  For the BQ76930 and BQ76940 the filter capacitors are large, and if the thermistors are used for temperature measurement they can pull on the supply pin voltage and open cell net as temperature is sampled every 2 seconds.  Voltages may move oddly with the open connection, but a test with a cell balance should be easily observable.

At VC0 the pin abs max may not support the full cell voltage and may be violated by cell balance test with an open wire. Generally the battery would be considered to have failed anyway so this may not be a concern, but design a limit if needed.   If VC0 is connected at the board and the battery- connection breaks, cell 1 will have not voltage and current will flow through VC1, the part may still respond although abs max is violated.

The part has the UVMINQUAL parameter for the automatic configuration feature.  If cell voltages are pulled below this threshold quickly the part will not recognize the cell as undervoltage.  The host must recognize the problem in an open wire test.