unfortunately inherent dangers of using li-ion cells are not always apparent. cpf'ers are on bleeding edge of technology, using consumer grade loose li-ion cells.
greatest danger using li-ion cells occurs during re-charging, not during use. most lights are constructed well enough to prevent dead shorts. use bare li-ion in series with extreme caution. protected li-ion cells are always preferred.
some li-ion cells are indeed protected for high & low discharge. overcharge protection is where it's needed the most. Pila's contain overcharge protection. (According to AW all of his protected cells contain overcharge protection.)
if you are not going to invest in a hobby grade charger. safest li-ion cells to use is with overcharge protection.
new Pila chargers will terminate charge at 4.2v. most low end chargers will trickle charge after light turns green. a trickle charge will eventually overcharge.
li-ion cells have a narrow charge widow of 4.10v-4.20v, without a natural termination point. And will accept charge long as you feed current to cell. until thermal runaway is reached... rapid disassembly of cell (explosion) if other cells are close by (secondary explosions) possibly burning down your home.
it's not possible to charge li-ion in series and stay balanced without individual leads going to each cell. This requires charger with multiple channels or balancer module. safest way to charge li-ion cells is in singles! multiple li-ion cells can be charged in parallel. make sure cells are within .5v of each other to prevent excessive currents as cells start to balance each other. charge rate should be set to 1c of lowest rated cell in stack. http://candlepowerforums.com/vb/showthread.php?t=161299
personally I would not use low end chargers. Invest in a good hobby charger. I use schulze and triton. lots of other choices. it may be worth reading balance cradle thread. http://candlepowerforums.com/vb/show...ight=explosion
it's a relatively new trend to use multiple li-ion cells to drive hotwires. li-ion cells when discharged in series at high rates will become unbalanced. there's evidence dentrites will form during slow charge after cell reaches 4.2v and during high discharge exceeding mfg recommendations. these sharp formations could cause an internal dead short.
if you charge those same series of unbalanced cells. charger will charge up to correct voltage/termination point for number of cells. but internally some cells will be undercharged, some will be overcharged. if unbalance condition becomes bad enough you will reach thermal runaway (explosion).
C & D sized li-ion cells have been available in limited numbers with little guidance of dangers during re-charging. accidents with C/D sized li-ion cells will not be minor events. (AW's C li-ion contain protection PCB)
The RC community has been using and recharging lithium polymer packs for sometime. there's been a number of explosions documented. mostly during recharging. http://www.rcgroups.com/batteries-and-chargers-129/
So far we have only one documented instance of a li-ion cell exploding on cpf. this was caused by operator error, selecting an incorrect setting on the charger. http://candlepowerforums.com/vb/show...ight=explosion
charger involved in this explosion caused by user error is http://candlepowerforums.com/vb/show...highlight=norm
fortunately no one was injured and house didn't burn down. but results could have been much worst.
it's extremely hard to cause a NMH battery to explode. VS it's relatively easy to cause a li-ion cell to explode by overcharging.
until A123 type cells become available in sizes cpf'ers can use. it's my desire to raise awareness of inherent dangers of li-ion use and avoid series of explosions and fires RC community has experienced.
apologies for length of post. I've tried my best to avoid technical mumbo jumbo....for more details http://candlepowerforums.com/vb/show...ight=explosion
If any information needs to be updated or corrected. please let me know and I will get it done. due to evolving nature of state of li-ion cells. this post is subject to revision.
if I had to make a few recommendations for using li-ion cells:
DISCLAIMER/CAUTION: I am not a battery expert nor do I pretend to be one. Please verify this information for yourself. Use this at your own risk. Not responsible for anything. Information for consumer use of loose li-ion cells is constantly evolving.
1. recognize greatest danger of using li-ion cells occurs during re-charging.
2. invest in a hobby grade charger and avoid use of low end chargers.
3. use li-ion cells with internal protection circuits (overcharge, over-discharge). especially when used in series.
4. don't charge in series without balancer leads going to each cell. w/independent channel for each cell.
5. safest way to use/charge li-ion cells is in singles.
6. don't use li-ion cells in series without protection circuits.
7. beware of dead short dangers. li-ion cells discharge at high rates
8. never charge li-ion cells unattended
9. charge li-ion cells in a protected area, so if fire should occur. your home will not burn down.
10. discharge larger li-ion cells to 3.4v before shipping. this removes almost all stored energy in cell.