Originally Posted by
helicomatic
You can also damage a lithium ion battery from overcharging it.
Overheating a lithium ion battery can cause thermal runaway past a certain temperature, where the increased heat will cause more increased heat, until the battery breaks open, and catches fire or explodes.
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.