PTC strip monitor and charger shutdown circuit

PTC strip monitor and charger shutdown circuit
PTC monitor and shutdown

The mega charger can put out over 750 watts of power. If a really bad cell with high internal resistance is in the pack, it will get very hot very fast. Honda wa aware of that, and they built in the PTC strip with a resistive PTC sensor on each cell. These detectors will change resistance with temperature, and rapidly change from .16 ohms each to over 1200 ohms when hot.
Since we are not so much concerned with actual resistance as we are in resistance change, a simple resistance bridge circuit can easily monitor the resistance and allow us to set two thresholds of alarm. The first alarm will be set to turn on an LED to let us know that things are getting warm, and the second will unlatch the power relay that supplies the charger with AC power.
The only issue is that the dual charger ties the 12V fan supply negative to the HV negative to allow measuring the pack voltage. I suspect that the BCM would not like the pack negative to be connected to the PTC circuit, so an isolated 12V supply should be used for this circuit, or the PTC connector should be removed from the BCM.
PDF of PTC circuit