Why do battery cells fail in groups?

Question

I have been observing for a long time that battery cells seem to fail in groups that are in close proximity.

I agree that observing 3 cells in a flash-light is not a very ample set, but I recently had the chance to observe several boxes with close manufacturing dates that have held sets of about 100 cells each (in close proximity) and in which several failed. All batteries with expiry date way into the future:

In all sets one could observe one battery failing bad and the others next to it following in the same path, as if one was acting as catalyst in the process.

Is it known why this happens in this way?

Answer 1

several boxes with close manufacturing dates that have held sets of about 100 cells each (in close proximity) and in which several failed. All batteries with expiry date way into the future

Properly stored Alkaline cells should not fail, so if you have several that did then there is a problem - perhaps defective manufacturing (fake cells?) or improper storage.

However as the saying goes "One rotten apple spoils the whole barrel". An alkaline cell is more likely to leak when discharged, due to build up of hydrogen gas which makes it vent. One cell may have high self discharge, or the electrolyte might eat away part of the case, or the seal might be defective. In either case the result may be electrolyte leakage. The leaked electrolyte is conductive, so it can discharge adjacent cells and make them leak too. This may be why you see clusters of failed cells.

Due to the increased chance of leakage, alkaline cells should not be left in a device when they are nearly flat. Some devices draw a small current all the time (even when 'off') eventually discharging the cells, with a similar risk. The worst case scenario is if the device is accidentally left turned on, when the chance of leakage is much higher. Therefore it is best to remove the cells from the device before putting it into storage.