Just to follow up on this... One of my "other" hobbies is radio controlled airplanes and helicopters... and when Lithium Ion
batteries came along they almost revolutionized things. Transmitters, receivers and now even electric powered machines... These cells are unprotected... Meaning they CAN be discharged down to lower than safe levels... Someone unfamiliar with their chemistry might not fully understand that, and I won't go into boring glaze your eyes over details, but here's the bottom line... It's not the CHARGING of a battery that's actually unsafe... Though that is where the consequences are suffered... In fact, it's the discharge of these cells that can create the danger. With a Lithium Ion battery if a cell is allowed to discharge down below a safe level (most use 3.3 volts, but realistically speaking 3 volts is where it really happens.. That's 3 volts PER cell if you have multiple cells... Anyway, once discharged below that "safe" level the battery itself won't show any signs of damage... It won't explode or steal your car.... It'll look perfectly normal... However, when charging it again that's where the magic happens...
To demonstrate this, I'll share a personal lesson.... I had a mid-sized electric heli called a T-Rex.... In it I had a battery that was rated for tremendous current drain... I believe it could handle 70C... It was a 2000mAh pack if I recall correctly, meaning it could discharge at 140 amps without damage!!! My charger used a temperature sensor that would sense the temperature of the battery during charging and if it detected the temperature rising too fast it would stop charging to prevent any serious issues.... I always made a habit of making sure the cells were above the safe minimum before charging and always placed them in the middle of my slate pool table during charging figuring worst case scenario if the worst happened it would just burn the felt on my table which was easily replaced... One night I was in a hurry to get to bed... It was very late and I wasn't paying nearly as much attention as I should have... Because of the "safety measures" I employed I never thought twice about letting a pack charge over night or while I was away...
That night I made a mistake... First, the pack had been discharged for a while, and I didn't check it first... So I'm 99% positive it was below the 3v per cell minimum... Secondly, when I hooked up the charger I didn't notice that the temperature probe had come a little lose from the charger and despite it having a warning flag on the screen I paid no attention to it...
I just went to bed....
Until about 2 hours later when I woke up to the smoke alarm going off and a toxic stench lingering in the air... I ran out of the bedroom to invesitgate and found the entire house filled with terrible acrid smoke... When I made my way to the billiard room I saw what could best be described as a firework display... It looked like one of those ground fountains had been set off right in the middle of the pool table... Streams of sparks and molten metal spewing out...
I yanked the cord from the charger and started opening the windows... When the smoke had cleared and the fireworks had ceased I investigated... The entire felt top had been burned away... Molten metal had spewed everywhere... Even beyond the confines of the 9ft tournament table and down around the sides... Chunks of metal had burned holes into the carpet....
It was my own stupidity... and I learned a very valuable lesson that night... Even when you think you're being safe, you probably aren't... Thankfully I did have it on that pool table... if it hadn't been there is no doubt it would have set fire to whatever was near it... the whole scenario could have turned out very differently... I think about that night every time I plug in a charger with 18650s or even my old ego sticks... Protected or not crap happens... even under the safest of conditions... So I always charge when I'm near by... and always try to be as safe as possible... I now have a "crock pot" that I sit my charger in during charging... Just in case... cause you just never know when the worst might happen!!!
and walks away shaking her head...
