I cut off the the outside package of the these 2 bike light battery pack, just to see the difference between them. 1 is 8.4v, 1 is 4.2v.
I kind of played them in my hand, and put 1 on top of the other. just in second the heat came out from the cell, and there is smoke.
my hand got burnt but just hurt a little which is lucky. :_(
the thing that surprise me is that it happened so quickly. just in one second!!!
Woah, I’m glad you’re not injured.
Was one cell reading 8.4V? That’s pretty dangerous!
You know a cell cannot reach such voltage, right?
8.4V are 2S2P packs...
Yeah, that’s what I thought. 
Did you put two battery packs on each other?
I’m glad you’re okay!
playing with Li Ion is just like playing with grenade.
be very careful 
yes, I knew if you put 1 battery with wrong polarity when there’re 2 cells in the light case., then there could be a leakage as one cell may charge the other. In my case, I don’t know it could be so fast. I would say WOW!!!
Uh yep, lesson learned eh? Glad yer ok. I learned a lesson not long ago. Tried measuring a C cell from top to bottom with calipers. Man, that battery is warm! Didn’t realize right away that I was shorting the battery with the calipers! Doh!
Yeah, I think what happened was you shorted one of the 2s2p back on the parallel connectors on the other pack. I’m glad you noticed the short right away!
I can’t ignore it, since it burnt my hand already and give a smoke. you can see the metal changed color in my picture because of the heat.
and I have to wash my hand, since feel tiny little hurt, guess should be the leakage caused.
this really just happened in seconds.
It’s much more dangerous than that, even if you have the cells connected CORRECTLY, if one is not balanced with the other (say one is full 4.2v and the other is 4.0v) the lower one will drain first and then reverse charge from the other and boom! Even with both cells installed correctly and the light working.
Your lucky all you did was short them out temporarily, they could have literally welded themselves together and been stuck like that till they blew.
I’m not trying to be harsh on you but Li-ions are unforgiving, do yourself and your family a favor and if you ever do mess with them again you need to do it on a non flammable work surface with and have a bucket of sand near by to toss them in should something go wrong, water, EVEN FIRE EXTINGUISHERS, won’t put out a lithuim fire, the ONLY thing you can try is to smuther it in sand and that’s no guarentee.
Water will only wake it worse, Li+H2O= ignition.
For future reference you don’t need to take them apart to know the configuration, if a 4 cell pack is 4.2v its 4p, if its 8.2 its 2s2p, if its 16.8 its 4s. Cells in series = add voltage, cells in parallel = the same voltage.
The same thing happened to me when I scavenge 18650 from a laptop battery… Its really scary and dangerous ! Its like defusing a bomb !
Yeah, be careful with those unprotected bike packs. The cheap/ebay ones are usually made from laptop pulls. The better made from new cells might have protection, but it’s not rule.
The smoke came from the wires indeed, no biggie, even the tiniest lipo can burn you if shorted. But its the wire that burned you, not the battery.
I’d measure the resistance of the burned piece and leave it like that if it was ok or solder some wire over or next to it if it wasn’t avoiding heating the cell’s bottom much.
The most important thing is to know HOW you shorted it and prevent it so it cannot happen again.
You should also rewrap those pack before using again of course.
well… i have one instant with one of my battery pack similar to OP, when i was charging it, it makes a beeping noise, it won’t charge…
i thought the battery pack were dead, i cut it open and test them one by one, turn out 3 of them is good and 1 is dead but
it didn’t explode or smoke… it simply won’t charge, when one of the battery goes kaput, the PCB on that battery pack kicked in and simply refuse to be charge and stop them from charging each other.
unfortunately the horror story that i heard so much about lithium battery is usually was mishandled or abused or over charged
( my iphone exploded after i forgot to take it off from the charger for 3 months after i got a new phone )
if lithium batteries are really dangerous, i don’t think they will be available so widely.
and just like anything… Cars, Guns, Lions are dangerous if mishandled or abused 
Actually, water puts out a Li-Ion fire quite nicely. Ask me how I know this.
You are referring to a water and lithium metal reaction, which would not be good. Fortunately there is no lithium metal in a Lithium Ion battery.
The short that appears to have occurred was the 2S2P pack shorting on the metal tab on the 4P pack. This would had been a lot of current, and that is a big tab. A smaller tab would have vaporized. If the cells have internal PTCs (they better, but no visible way to check) those would probably have opened after a few more seconds and stopped the current flow. Most reputable cell manufacturers have a PTC in there which open when the cell temperature reaches dangerous levels.
just want to say, I read all your comments, and it’s really helpful, and clear things out for which I’m not sure
BLF is the best. 0:)
Thank you for the report H.G.Sun. This is very good. It helps remind all of us that we need to be very careful with these batteries.
...and wedding bands are very good conductors...and yes, it only took a second to realize the battery wasn't completely dead.
Contact the distributor or the manufacturer. Perhaps they can tell what’s actually wrong there. Or they might give you a refund.
actully I’m the distributor. just unpack the battery to see a how. Since I know so little about them. the lessons here i learned is, you shouldn’t dispack a battery, any battery unless professional. and PCB protected in single cell is better than that outside the battery.