A Japanese safety institute has released a video that serves as a graphic reminder of the danger posed by damaged Lithium Ion batteries.

The batteries are extremely common in portable consumer electronics, providing power for smartphones, laptop computers, smartwatches and many other devices, and are typically safe. But if a battery is damaged, the results can quite literally be explosive.

As part of its work testing electronic components, the Japanese National Institute for Technology and Evaluation (NITE) tested batteries about the size of those used in cellphones. They were struck with a hammer then left on a work bench, unconnected from any apparatus.

Later—the period of time is unclear from the edited footage—one of the batteries ruptures with a bang, flying across the laboratory. A second clip shows a similar battery erupting in a shower of sparks.

The hammer blow to the battery caused deformations in the internal structure of the battery, which led to an internal short circuit. That created heat which sped up the reaction leading to more heat. It’s a vicious circle called a “thermal runaway” that results in fire or explosion.

Thankfully, such explosions are rare. Some are due to manufacturing problems, like those that affected millions of Sony laptop batteries in 2008, while others are due to damaged batteries. The recent fire of a Tesla car, which packs hundreds of Lithium Ion cells, was blamed on damage caused by road debris.

The advice from all battery manufacturers is the same. If your battery sustains any kind of damage, replace it.