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When the energy shoots out the positive terminal, it will heat up anything in its path. If there is another cell in its path it will heat that cell up and cause it to go into thermal runaway, which causes a chain reaction if many cells are connected in this way (thermal runaway propagation). If however, we remove all the cells from the path of this thermal runaway energy, we should be able to reduce the chance of thermal runaway propagation. This was the motivation to not have any cells stacked vertically on top of each other.
Note then in the final module design, we had an acetal sheet placed on top of the cells, and some cells in the module had the positive end facing down, so there is more resistance to gasses trying to escape from the positive end since there are more things in the way. We also used a potting compound to cover the spot welds, and decided on a semi-flexible compound instead of a rigid compound to give less resistance to any TR ejections trying to escape the top of bottom. We did not test any cells going into thermal runaway, but my guess would be that there would be some cells that blow out the side and cause thermal runaway propagation due to the increase in resistance out of the positive end compared to in free air as the tests in the NASA research were done.
Another great presentation on TR propagation minimization:
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