Thanks again Bob,

I would agree with your explanation.
So the failed cells had no electrolye solution left whatsoever? Or enough of the separator was exposed that it was not cooled. The poor efficiency of the used cells 30-60% shows that there is a non-conductive/high resistance film present on the electrodes which will heat the electrolyte solution leading to water blow off from the pressure release valves. Keeping the electrolyte topped up should prevent a "hard" failure. I think you would still need periodic reconditioning/restore by the method you showed to extend the life of the battery.

Great work
Thanks
Andrew

Andrew