[Coda]

Short-circuiting batteries ARE dangerous. The smaller the battery, the less of a problem it is, but the current flowing without any sort of load to slow it down generates heat, and the heat causes the battery to degrade. The chemicals inside the battery start to react more vigorously, generating more heat. The heat causes the battery casing to weaken, AND the chemistry starts to produce gas (either as a result of the chemical reaction or as a result of the liquids boiling), making the battery swell. If the casing ruptures at all, then it leaks out caustic chemicals that can burn skin and corrode plastics and metals.

For lithium batteries it's even worse, because an external short-circuit runs the risk of creating a bridge of lithium metal deposits INSIDE the battery, at which point there's no stopping it -- it becomes a runaway cascade, building up more metal that makes the short circuit worse, and the whole time it's making heat and hydrogen gas until the whole thing melts down, catches fire, and destroys everything nearby.