Thermal runaway

A lithium cell stores a lot of energy in a small space. If one spot inside gets too hot, the chemistry there releases more heat, which heats the next spot. The loop feeds itself and the cell can vent smoke or catch fire.

The FAA says this can begin without warning, including in cells that were simply built wrong. That is why spare batteries must travel in the cabin, where crew can see and reach them.

At home, the signs are a case that swells, a sharp smell, smoke, or a pack that feels hot when nothing is using it. Stop charging it, move it away from anything that burns, and have it recycled.