L9963E failure hypothesis: intermittent open connection on C13 sense wire during external balancing

[Dimchin]

Hello,

We are investigating a field failure of a BMS board based on the L9963E cell monitoring IC.

The system uses a 14-cell stack. Passive balancing is implemented with external balancing resistors and MOSFETs. The balancing resistors are located on a separate resistor board. The balancing resistor value is 10 Ohm / 3 W. The external balancing MOSFETs are BSS308PE P-channel MOSFETs (and PMV55ENEAR N-channel).

After the failure, we found damage mainly in the upper cell channels, especially around the C14 / C13 / S14 area. Several 100 Ohm series resistors in the cell input/filter network were damaged, and one of the balancing MOSFETs (VT3) was shorted.

We also found a mechanical/electrical issue in the battery harness: the C13 sense wire connection is intermittent. With the harness connected, the C13 contact sometimes exists and sometimes disappears. The failure reportedly occurred when the vehicle hit a bump, so vibration-related contact loss is possible.

Our hypothesis is the following:

1. Balancing of the cell connected to the C13/C12 area was active, or had recently been active.
2. The C13 sense wire temporarily opened due to the intermittent contact.
3. The C13 node on the BMS board lost its real battery potential.
4. Because the balancing MOSFET was on, the floating C13 node could be pulled toward C12 through the 10 Ohm balancing resistor path.
5. As a result, the L9963E could see approximately 0 V on C13-C12 and approximately two cell voltages on C14-C13.
6. This may have caused the internal protection/clamp structures of the L9963E to conduct, leading to high current through the input resistors and the external balancing circuit.
7. This could explain the damage pattern around C14/C13/S14 and the shorted external balancing MOSFET.

Could you please comment on whether this failure mechanism is plausible for the L9963E?

In particular:

• Can an intermittent open-wire condition on an intermediate cell connection such as C13, while external balancing is active, lead to destructive current paths through the L9963E input or balancing pins?
• Is the open-wire diagnostic intended only to detect this condition, or should the IC also survive such a condition without external hot-plug/open-wire protection?
• Should balancing always be disabled before and during open-wire diagnostics?
• Are there recommended external protection circuits or design rules for cell sense lines when external balancing resistors/MOSFETs are used, especially to survive contact bounce or hot-plug events?
• Are there any specific recommendations for protecting the Cx/Sx pins against this type of intermittent harness fault?

Any guidance or references to the relevant application notes would be appreciated.

Best regards, Dmitrii.