LOL! Yeah, we do love our regulations here. Mind you, I think we have fewer lawsuits and economic meltdowns than some of our neighbors. Too soon? 
I kid, I kid. Really just writing this for a friend. It’s just easier if I can send him a link instead of thinking of something every three minutes. He was interested, so I figured I’d do it up.
Imagine if he bought a bicycle, certainly more dangerous than anything this hobby has to offer, would you write him a cautionary note of everything to avoid?
The reason why I feel this urban mythology endures is that the “chemical” component of li-on’s creates almost an aura of mysticism that hot water or falling off a bike lacks. It’s the similar reason why people fear “terrorists” or other mysterious forces over far far far more common methods of demise.
Oh, I totally agree with the probability argument. In fact, I wrote a paper on post 9/11 arguing that the best response by the US was to do little or nothing. I’ve since kinda changed my tune (due to what may or may not have been the economic consequences of the attack). Nonetheless, there are other far better ways to spend government money bubble-wrapping the world, than on boogey man detectors and rocks that prevent bear attacks.
A five second read won’t kill anyone, though, is a nominal waste of time (as opposed to a course from battery university), and may have a slim chance of preventing something unlikely, but preventable, from happening.
If my friend was mildly retarded and not familiar with bicycles, I might write such a guide. Of course, the safety precautions are obvious to us. But to others who have no experience with rechargeables, common sense is almost nil; they just don’t know. It’s hard for someone who is so used to knowing what he’s doing to see just how much “common-sense” other people don’t have. It’s like trying to explain how to use a computer to a 90-year-old who has never used one before.
Anywho, methinks it’s time for bed. Cheers for now! 
As another example of how selectively overblown this whole li-on scare is, consider that for the longest time all that existed in “tactical” light land were 2x cr123a xenons. Multiple unprotected cells in high draw config almost certainly never tested before or during use, yet CPF gushes up and down about how solid (ie “pipe bomb”, ;)) their favorite surefire is. But sure as dawn when inexpensive li-on powered lights came about, even single cells with protection were the new terrorists in town (unless you used a certain brand cell with certain brand charger that is).
Check that the batteries are protected for a start. You should be able to detect a separate disc under the shrinkwrap at the negative end.
Best case scenario is they will be protected and have an actual capacity of around 1500mAh.
Imagine you bought a bike powered by Li-Ion 18650 batteries.
ROFL!
Sorry I missed your question… the one legit question this thread has had, so far (embarrassed)
It’s tough for us to say, really. It’ll depend a lot on your background knowledge and how that translates into the care with which you use them. What we CAN tell you are some of the worst-case scenarios (fire, venting, and theoretically, small explosions). Moreover, we can tell you that these events are RARE, and shouldn’t discourage you from trying cells if you exercise good safety precautions. The cells themselves are less likely to be the problem. If you’re prepared to wait a little bit, I hope to have this guide completed before the end of the week. That should answer a lot of your questions.
As for the quality of the cells, the experience of forum members is that with the cheapy cells, you sometimes get duds, have cells that are nearing the end of their life expectancy, and will in all likelihood, have less than the rated capacity (sometimes MUCH less). I probably did worse on my first purchase, and I’m still here, so don’t worry too much. I, personally, do not have experience with the products from this seller, so I cannot say what you’ll end up with.
(can’t sleep… clowns will eat me)
edit
+1 on checking to see if they’re protected. Don’t use unprotected cells in multi-cell flashlights. Also, ensure that both batteries being used in the flashlight are charged to the same voltage; if there’s a difference, charge them to match before installing.
It would almost appear the CPF opinion that the 18650’s in your bike light is more dangerous than riding. Better invest in a blast containment chamber for the battery pack rather than a helmet.
That’d be hard, but you can buy a car powered by Li-Ion batteries - the Tesla Roadster has ~7000 Panasonic 18650 cells!
Hello again, thanks all, and thanks sintro, Gimpy and torchythebatteryboy for the replies…
I don’t think the batteries I ordered have protection, I ordered them to use with TrustFire T1 (uses 2 x 18650 in serial connection)
I checked all the ebay and other websites they all sell flashlights that uses multiple li-ion with cheap batteries without protection (they even claim 4000 mAH) in bundle with real cheap chargers. If it is dangerous how and why they sell these?
Because they’re not.
Cheap doesn't necessarily mean that they are bad. My beef with Ultrafires is that they are so inconsistent in quality. You might have a decent charger and cells in your hand, but without the right testing equipment you will never know. Not everybody has the equipment, time, money and endurance that HKJ and old4570 have, to really test out the pro and cons of cells and chargers. If you want to continue to use those cells and charger, I would at least invest in a cheap but good DMM so that you can check voltages to see if cells are overcharged or too empty to use them safely. In a dual set-up like 2X18650 the same charge level is also important. My biggest pet peeve with unprotected LiIon cells is that something like leaving the light on and forgetting to turn it off can already change the chemistry to the point where it would be unsafe/ not recommended to recharge the cells at high amps. That is something that non-technical people still have a hard time to wrap their mind around ("When it's empty, I simply recharge it. Where is the problem?")... and when I do try to explain the effect on the chemical level, they look all funny at me.
The best overview of the potential risks that I found so far is at Battery University. But like other people already mentioned, these articles already are more for tech-savvy people or "beginners that are willing to learn". I don't like the thread title in particular, because "Idiots", or people that won't bother to invest some time in reading about battery safety simply shouldn't use LiIon cells in the first place. They need electronics that do all the thinking for them, like in cell phones and notebooks.
My gawd man, just give it up !
You’re like a dog with a bone. You’ve made your point eight times, now please move on and let the OP do what he wants to do.
So when are you planning to get around to criticizing the CPF + wannabe crowd for repeating ad infinitum blatant ignorance on this topic well before I was even around? Perhaps one reason why I might need to repeat a point is that apparently some have poor memory, such as when I said OP can write what he feels is right. I only hope others such as yourself can find the basic courtesy to extend the same to me.
I gave one of my friends a 14500 for a UfH2. I told him when it goes flat tell me and I will charge it for him or he will need to buy a charger. He said he will charge it in his nimh charger. I told him not to and explained why. He looked doubtful so I took the battery back of him and told him to stick to nimh. Even reading a thread like this he would still try it.
Yup, there’s very little in the way of safety information provided by the sellers of this stuff. I don’t know why. Some sellers will mention a few safety precautions, but might not give you everything you need to know. I guess most sellers assume that the people who are buying this stuff actually know what they’re doing, and will take responsibility for it themselves. I suppose, also, that some of them just don’t care.
Agenthex can stick to his conspiracy theory regarding CPF, but here’s what an ACTUAL manufacturer has to say about their product:
In general, it’s considered poor marketing to claim that your product can explode and catch fire if, in fact, it will not. And somehow, I doubt Panasonic was heavily influenced by propaganda on CPF.
Gimpy, I hate to burst your bubble, but the same dire warnings on product safety and misuse can be found on almost any consumer product sold in the U.S. Why? They have lawyers working for them and somewhere, at some time, someone got sued.
Li-ion batteries that we use in lights, like many other chemistries, can be dangerous if misused. Learn about them, respect what they can do, treat them well and life will be good. Agenthex has a point: I ride my bike 5-6,000 miles per year on the road. Believe me when I say I am much more worried about the guy riding 1 foot off my front wheel or the 2 ton car following behind us than I have ever worried about the 18650 battery in my flashlight.
By all means, keep writing and definitely have a section on battery safety, but try to keep it in perspective. You know if you drop a few Mentos in a liter of Diet Coke it will explode..... 
I am sure about the danger of all li-ion batteries, if you google you can find news everywhere exploded phones or other devices also 18650s…
I don’t know anything about the electricity or electronics but I am totally agree now after some research, if they are used in serial connection without protection the danger I guess doubles or maybe more.
I want to use this flashlight while cycling so I don’t have time or equipments to test them all the time, I just want to use them charge them again and use again, also I see myself really stupid (or idiot like the title of the topic:) so I really do not want a bomb on my bicycle near my face.
Can anyone please recommend me a normal priced (not very expensive, if possible cheap:) 18650 with a fully working pcb protection to use 2 of them in serial connection?
I found this: Boost - Buy In Coins
What do you think? Are they original?
Most of those sellers are in china. So they don’t really have a risk of getting sued like in the United States. I myself used to look at them, but they just seemed to good to be true, so I did some reading for a few weeks on CPF, and then found out about this site.
The highest capacity 18650 right now is the Panasonic 3400mah cell. You actually have to charge it to 4.2v though.
Basically, those sellers sell fake cells and bad chargers just to make some money! Now, they AREN’T super dangerous. I’m sure they would get alot more scrutiny from everyone if every single person bought those crap ebay cells and chargers, like they do aa alkalines. There would be many more injuries from them, and the lawsuits would follow.