I liked the look of this light, so I checked out the Amazon link that was in the initial post.
Jazzy - By chance, do you have a multimeter? I would recommend that you check the voltage of the batteries (1) after this device has fully charged them, and (2) after this device has shut itself off due to low battery. While doing this test, I would recommend doing the charging only when you are home, possibly even outside.
Why am I recommending that you check this? Because I found this light only costs $19.97 And that includes the light, two 18650 batteries, and a built-in charging circuit. They claim the device is 5200mAh, so that would be two 18650's at 2600mAh each. This is an expected capacity for lower end ICR 18650's, so not a problem itself. However, a 2600mAh, protected, 18650 from a reputable manufacturer is going to cost around $12 each. That's buying one individually, not included with a product, when they would probably be cheaper.
What I am worried about is just by the price of this thing, you are possibly dealing with cheap 18650 batteries, of unknown manufacture, that may not have any protection circuitry in them (even if they were to claim they did). Unfortunately, this is a pretty well known problem for some questionable Chinese stuff. Note that most batteries and chargers come from China, even the high quality ones, it's the no-name junk stuff that I am talking about here.
You want the fully charged voltage to be a max of 4.2 volts (not higher). You want the fully discharged voltage to be no lower than 3.0 volts. Also, when the batteries are charging, reach in there and touch them to see if they are hot. That's a bad sign. Slightly warm is one thing, hot is another. I would not expect your batteries to get hot, because at 2600mAh capacity they can safely be charged at up to 2.6 amps of current. And the little USB plug-in chargers that this light would use don't supply that much current (only "quick chargers" will, but they limit themselves when not connected to a quick charge compatible device). So bottom line, if your batteries DO get hot - you've got a problem! Unplug things immediately. Question: "If the USB charger won't supply that much current, why would the batteries get hot?" Answer: "Because in typical Chinese electronics tradition, they may not really be 2600mAh batteries - they may only be 1000mAh despite false labeling. And a USB charger COULD supply enough current to overheat these smaller batteries if a cheap charger without thermal sensors is in the mix."
Anyway, for your ultimate safety and your (and my) peace of mind, it would be nice to know that the charging and discharging in this light is not putting you at risk of a Li-Ion battery explosion or fire. Overcharging Li-Ion batteries above 4.2 volts can blown them up in your face. Over discharging them below 3.0 volts you may be able to get by with a few times, but it will be damaging the battery, until it finally won't charge or worse, become a fire hazard when you DO attempt to charge it (with a cheap charger).
A "protected" 18650 (or any size Li-Ion battery) will have internal circuitry that keeps it in the safe 3.0 to 4.2 volt range. A quality charger will also keep the battery in this range by automatically shutting off when the battery is full, and refusing to charge an over discharged battery. Some good chargers will recognize an over discharged battery and attempt to revive it using a low trickle charge. But many chargers will simply refuse to charge one of these batteries and flag it as being bad. High end devices that are designed to use these Li-Ion batteries will also shut themselves off when the voltage gets too low. It is all these layers of protection that keep us safe when using rechargeable Li-Ion batteries, which are inherently pretty nasty things. When one layer fails, or is fraudulently claimed to exist when it it doesn't, the hope is that a different layer will save our butts.
Cheap Chinese chargers sometimes depend on the batteries protection circuitry to stop the charging process when the voltage gets too high. And some cheap Chinese batteries don't contain a protection circuit even though they lie and say they do. Mixing a cheap Chinese charger with cheap Chinese batteries can be an explosion/fire disaster.
I'm not trying to scare you here, I am just concerned about your safety. Basically because the cost of this light is quite low for what it contains. THIS DOES NOT MEAN THAT IT IS BAD! It just means that if I get one, I will at minimum check the batteries with a multimeter as described above. Or remove the batteries it comes with and charge them in my known good quality charger outside of the light. Or replace the batteries with high quality ones (which seems silly, since that would cost more than the light itself did).
If you do buy replacement rechargeable Li-Ion batteries for some device at some point in time, please buy good quality ones from respected manufacturers, that indeed do contain the protection circuity that they claim. Stay far away from things on Amazon named "TrustFire", "UltraFire", etc. Basically anything with the word "Fire" in the name. That is the Chinese trying to confuse you into thinking these come from the reputable manufacturer "SureFire", which doesn't make rechargeable Li-Ion batteries BTW (they didn't used to, maybe they do now?) They might as well name these Chinese things "GuaranteedFire", because they are almost that. Well, maybe that is a bit of an exaggeration on my part, but you're certainly "PlayingWithFire" there! I would recommend always buying a name brand reputable charger. They only cost a few dollars more than the Cheap Chinese Crap stuff. Then you can maybe get by inserting a cheap battery into it. But never get a cheap charger AND cheap batteries. I would recommend buying NiteCore branded stuff - both the batteries and the charger. They have an excellent reputation for safety.
Sorry, I kind of went off the deep end on explaining and warning about this stuff. I have never had a Li-Ion battery fire myself. You wouldn't know that from my level of paranoia. We were only talking about a rechargeable light to begin with. But that light has got some pretty powerful batteries in it, two of them (and more-than-one is more dangerous than only-one for reasons I'll shut up about now) and it's quite inexpensive, so ... I worry a bit.
