I was on the old good-for-nothing YouTube the other night and I found myself watching videos of people poking Lithiun ion batteries with kitchen knives, causing them to combust in huge piles of fire.
So, I rotate L5 batteries frequently as a way to keep charge. Sometimes I carry them around like candy. At least once I had one in my pants pocket with keys, and the keys scratched it up.
Was I inches away from making a giant fireball with that key, or would L5 batteries not do that?
There’s nothing L5 specific about that. If you mishandle your batteries, they may fail in interesting and aggressive ways. Fireball not guaranteed, though. If your keys touch the heads of the battery, it can short, which would lead to heat, which would lead to fire (of the battery or something more flammable first), which may lead to fireworks (regular or more interesting)… but at that point you maybe are already injured at the pocket area, so that should probably be less relevant to you.
The point is, you shouldn’t carry or store batteries loose with other items. A protective case for re-chargeables (just like with AA, AAA, 18650 etc.), or tape the heads if you are recycling them via a collection bin. We call them “ghost zombie battery fires” when batteries touch each other unprotected and burn the collection bin. On the other hand, especially if it’s big enough, it’s “runaway thermal event”. Some research on it.
One safety site about this: https://www.canada.ca/en/health-canada/services/household-products/battery-safety/lithium-ion.html
Edit to add: excellent footage of various small lithium battery fires - including some that happen in pockets: https://www.youtube.com/watch?v=8nz5ijXcckI (sorry about all the YT links). Also, iFixit has a good article about this: How Smartphone Batteries Can Catch Fire—and How to Prevent It - iFixit (interesting safetytip about using less than 25% charge).