CS:
@ Murphy...I was trying to make the point that, even doing it on the cheap with discrete components and maximum redundancy, it wouldn't exactly be overwhelmingly complex. Every single sub-cell could be monitored and contained thus giving a hugely enhanced safety-cushion.
I understand your were providing a "proof of existance" not a necesarily a final dessign.
Depending on location of monitor PCB, preferably outside the "fire box" so logging would survive an 'incident', serial bus sensors would almost certainly be cheaper than individual wires given the cost of certified connectors and wire harnesses etc.
Have to say I am very puzzled that the cells are not individually monitored in the current design since that would give the best early warning of problems.