Ok, so concerning lipo’s catching fire:
Lithium-Ion Polymer will catch fire if they get too hot. There are a few ways to do this:
- Over charge (current) - the excess current is turned into heat. The electrolyte solution may also not be able to absorb enough of the energy, and therefore also turn the excess into heat.
- Over discharge (current) - batteries have internal resistance, as the current increases so does the heat.
- Over charge (voltage) - as with current, excess voltage is turned into heat.
- Short circuit - same as over discharge.
- Punctured cell - same as short circuit. The foil layers within the LiPo will short against other layers.
Once the temperature gets to a certain point, the battery will go “critical” - this is a self sustaining chemical reaction which cannot be stopped. The cell decomposes rapidly, first puffing up, then rupturing to jet very toxic (and flammable) smoke, the internal temperature then reaches a point where that jet of smoke catches fire. At this point you no longer have a battery, but a rocket motor. You have violent eruptions of smoke and flame, and additional cells that may not have been initially affected by the heat are also now over heated. This increases the fire jet effect, as the flammable smoke from the other cells is also set on fire by the first. All in all, this can be either a very good evening, or a very very bad one depending on your intentions.
LiPo’s that have decomposed can fly several metres under their own thrust, spewing fire and some all the way, happily setting fire to everything in sight. This is an excellent way to burn down your garage, house, or car depending on where you were charging it!
Additionally, a cell discharged below 2.7V per cell may not be able to accept charge again, one below 2.2v/cell almost certainly will not. At this point, charging it is the same as pushing current into a fully charged lipo - your LiPo transmutes into a rocket motor!
To avoid all of the above, use a charger designed for the chemistry of cell you are using. DO NOT use a Lithium Ion (like what’s in older electronics) for Lithium Ion Polymer (very bad idea). Do not use a Lithium Ion Polymer or Lithium Iron on a Lithium Iron Phosphate (LiFePo4) battery. Above all, do not use a lead acid or NiMH/NiCd battery charge on a lipo cell unless you’re looking for fireworks.
As a fun fact, a 2200mAh 3 cell lipo contains a similar amount of energy density to a stick of dynamite. It releases it over 2-3 seconds rather than 0.1s, but it still a violent decomposition.
Basically: Use a charge circuit or IC designed specifically for the battery you are using - they are perfectly safe if you treat them properly and with the respect they deserve. Shooting them as in the video of mine Chris linked does disappointingly little, however i wouldn’t recommend charging a cell after doing so.
