Originally Posted by
GlowBoy
Absolutely right! NEVER use or charge a Li-Cobalt battery without protective circuitry in place! There are a number of conditions (undercharging, overcharging, excessive charge current, excessive drain current, physical damage, short-circuiting) that can cause a Li battery to overheat and go into thermal runaway. Li battery fires are extremely powerful and intense, can also be explosive, and also spew extremely toxic fumes. That circuit board is designed to minimize the chances of these things happening. Sure you can probably get away without using it, but it's a bit like driving without a seatbelt.
These dangers are in fact exactly why I've stuck with NiMH so far for my homebrew lights, and will go directly to Li-Manganese or Li-Phosphate for my next battery.
You are correct in that you shouldn't charge a lithium ion battery without a protective circuit and that there are a number of conditions that can lead to problems. It's not overheating that's the problem. Overcharging and overdischarging can lead to a reduction of the lithium from an ionic state to a metalic state. Metalic lithium (elemental lithium) is extremely reactive. It will react with the materials (water, usually) in the cell and release hydrogen. It has enough energy to cause the hydrogen to combust. Alkali metal fires can't be put out with water (the water reacts and forms more hydrogen and makes for a bigger explosion
). In labs where we use elemental alkali metals (sodium usually) we have to have D class fire extinguishers. These are filled with sodium chloride or a copper based dry powder that dissippate the heat and absorb the alkali metal.