This is Mount St. Helen, one day before it erupted. There is no smoke, no magma, just an eerily silent mountain visible.

Let's talk about 18650 battery explosions - what they are, and what they are not. I think an interesting and telling analogy to explain how a battery explodes is by looking at Mount St. Helens - the deadliest and most destructive volcanic event in the history of the United States (economic because it wiped out over 2 billion dollars worth of lumber).

In 1980, when it erupted, US geologists knew very little about volcanoes. At that time, no volcano had erupted in the lower forty-eight states in over sixty-five years. The only ones we had to study were in Hawaii, which we will come to see are an entirely different beast. St. Helens started rumbling on March 20, and within a week was erupting magma - albeit only in very modest amounts. It became a major tourist destination, with dozens of helicopter tours - no one thought it was going to blow quite like it did.

On April 19, the northern flank of the mountain began to bulge conspicuously. If you know about batteries, bulging means an explosion is imminent. This is especially true for lithium-polymer "soft-packs" like in old cell phone batteries. However, no one thought this signaled a blast in St. Helens, and tourism continued flocking to the mountain for pictures. No one except one person - a geology professor named Jack Hyde.

Mr. Hyde pointed out that St. Helens did not have an open vent, like most Hawaiian volcanoes do. Now, if you know something about batteries you have probably solved the puzzle. The pressure building up inside the mountain was bound to explode catastrophically. The next day, it exploded with the force of five hundred Hiroshima atomic bombs, creating the biggest land-slide in human history - enough displaced land to completely cover Manhattan.

What St. Helens looks like now - after the eruption.

Lava erupting from the Pu'o vent in Hawaii

Here you can see what happens when a volcano has an open vent. There is no giant explosion, just escaping gases and chemicals. The same is true for 18650 batteries - they rarely explode with a huge force of a bomb. More likely, they will release pressure by spewing dangerous chemicals and sparks out of a vent right underneath the positive terminal. If you look at a battery up close, you will notice a ring of holes that go through the top cap and lead straight into the can, or negative terminal. If these vents were not on the battery, failures would be far more devastating.

Close-up of the top cap, and vent of a NCR18650 BF battery.

So if there is one take-away point to understanding how batteries explode using a volcano - it's that you have to trap volatiles for an explosive eruption. Vents release pressure, and as long as you have a properly vented battery you are not in serious danger if it fails.