How does UL test batteries?
While not a formal UL test, the Blunt Nail Test initiates fault if a single cell fails due to a short circuit. If the positive and negative battery chemistries are separated, the chemical paste in the battery can heat up so quickly that the battery explodes. A properly-made battery contains internal protection for the cell. Even if one cell catches on fire in 24-cell battery pack, for example, a domino effect may transfer energy from one cell to the entire pack, called a thermal runaway. This means the entire pack would catch on fire.
The Projectile Test checks to see if the cell can withstand heat. For this test, if a cell fails and begins to overheat, the cell must be able to contain a heat reaction. A battery is placed in a wire screen, and then heated with a Bunsen burner. When the battery begins to react with the heat, it cannot penetrate the wire screen in order to pass the test.
View the video to see how UL tests lithium-ion batteries.