@Chris Scott.
Could you just clarify the last sentence of this part of your penultimate post, please (my bold)?
“Shorting
It is not going to be a solid short from one moment to the next. The energy is just too great. Cell could not ever dissipate it's 200 - 300 Watt hours quietly. It would short, arc, burn out the short and do more damage along the way. There would be over pressure, relieve valves opening, electrolyte blown out. More shorting as separators fail by the heat. The whole cycle continues until energy is dissipated and some spot shortened can not be burned out.”
I look at it at the cell level, from an electrical perspective, which I understand and which is the initial trigger of the destructive cycle. I disregard secondary effects of fire sustained by chemical reactions and thermal runaway.
A cell consists of stacks of Anode-plate Separator Cathode-plate. A short circuit occurs when any part of anode touches part of cathode. It could be caused by plates deforming or separator deterioration or contamination due to chemical reactions.
That process starts at a spot, the area of which is a quite small part of total cell area. Short results in a large current density at that spot leading to arcing and local heating. The effect of arcing is increased current flow and more heat. Once the spot is hot enough, it evaporates and the short may weaken. Also, resulting pressure increase may push plates apart and weaken or open the short.
However, the damage has been done and another spot will fail. 200-300 Watt hours is a lot of energy to dissipate within seconds. The short-arc-open cycle goes on until the cell is discharged or ceases to function for lack of electrolyte.
Also, do you think that a manganese (spinel) type of Li-ion battery would have been a safer choice than the cobalt type, and would its performance be adequate for the task? If so, could its retrofit be one of the options currently under consideration?
There are two problems on my mind, two separate things to resolve. 1) What caused the initial failure. 2) Was the failure mitigated by disconnecting the battery immediately from all power.
Changing cell chemistry or manufacturer would be effective only if 1) is caused purely by bad cell. IMHO, a cell management issue is much more likely than bad cells. The investigation will figure it out.
If cell management is the problem, "Safer cells" would not make a significant difference. Safer cells mismanaged would break too with quite similar effects due to high energy density.
What makes cells fail is imbalanced charge or load, it does not have to be the charger, it can be the bus, or the interaction of both.
IMHO, other batteries in service, will already show detectable symptoms of impeding failure.
What one could do is to inspect batteries from all over the fleet using CT to look for anomalies such as distortion or contamination and then track back to cell quality or cell management.