royfellows said:
18650 cells are Lithium Ion Phosphate which is generally the safest form of Lithium Ion battery. Lithium Ion Polymer is an entirely different chemistry and some batteries of this type have been known to explode if subjected to violent shock.
Very few 18650s are Lithium-ion phosphate (LiFePO4), the majority are Lithium-ion Cobalt (LiCoO2).
Phosphate is the safest form by far, but it has a lower operating voltage than the standard LiCo cells. You can not mix and match cells or chargers. The good news is that whilst phosphate cells are available to Joe Public, he'd have to go looking. Phosphate cells have a lower energy density, handle less current and are more expensive than their Cobalt based cousins.
Li-Polys are usually distinguished by their soft cases. They are the more dangerous of the group. They have the highest energy density and therefore the most potential to go pop. This is what you'll find in your phone, safely sealed away from prying fingers.
All Li-ions require a safety circuit to disconnect the cells if the voltage gets too high or too low, or the maximum charge or discharge currents are exceeded. This is the secondary protection circuit. The primary being that the charger / load itself shouldn't be abusing the cell.
All (reputable) 18650 have tertiary protection inside the cell. A over current / over pressure device will permanently disconnect the cell and vent gas if the cell internal pressure is too high, or if the cell is short circuited. I wouldn't like to vouch for the reliability of this on clone cells.
I've seen nails driven through cobalt 18650s to test their safety (do not try at home). I've seen them crushed and heated. They're quite resilient.
Short term immersion shouldn't be a safety issue to a 18650. It may cause the cell to over discharge, it's certainly not going to do it any good. I'd be worried that long term immersion could compromise the internal safety device.
Yes I'd happily remove a discarded one from a cave, but I'd keep it where it could be thrown away if it started getting hot.
Roy's absolutely correct in his advice about the plastic sleeve. It's not wise to rely on that alone as insulation.