Last night, my wife and I were awoken by two loud cracks a second or so apart. The day after, I discovered that the 9-volt battery cell I'd removed from a fire alarm the night before appeared to have exploded (see photo). What might have caused this in a depleted cell, and why two cracks? This photo shows a "battery" in the real sense, being constructed of six 1.5-volt cells connected in series. Some batteries have stacked flat cells but others, like the one pictured, have a three-by-two array of cylinder cells.
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