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I thought this was a neat product some here would appreciate, especially those who already have unprotected 18650 Lithium Cells and a proper charger for them.

18650 cells are the batteries that Tesla currently uses in their electric vehicles and are the, or very close to the most energy dense lithium cell currently available.  Many laptop batteries consist of 6 or 9 of these cells in series parallel.

Often when laptop batteries are worn out, it is just one of these cells in the battery pack which is no longer upto snuff, and the BMS removes the series of cells from Duty, and the battery pack instantly loses 1/2 or 1/3 its capacity.
This was the case with my former 9 cell battery pack.  When I opened the battery and extracted the cells, only one of them refused to hold 4.2 volts when charged separately, individually on a Nitecore i4v2 charger, the 8 other cells still have a good portion of their original 1900Mah capacity left.

Lithium batteries can be dangerous if misunderstood and abused so those not familiar with their proper use and care should begin to read up here and click some links within this thread, if interested:

Anyway, I just received this device today from the slow boat from China.  One of the reviews on Amazon, is mine, so I will not repeat it here.

I plan on using this when I have not fully charged my Bluetooth speaker or Phone and am using both in my Tool belt to keep my tunes playing through out the time I choose to work. There has been a time or 2 when i worried my phone battery would not last for the duration I would be away from a charging source, and since this device is small enough to carry easily, it could come in quite handy.
 I have 8 18650  cells from my laptop extraction, and 2 other higher capacity unprotected PANAasonic NCR18650b 3400mah cells to employ.  These panny cells should be able to fully charge my phone battery from dead, ~1.5 times, though I have not yet tested that claim. I am not yet sure how much capacity these particular cells have left as they do have a hundred or so cycles on them already, and I tend to charge them fully and leave them fully charged ready for use, and unlike lead acid batteries, this is not good for 18650's

The Panny cell which powered this device and took my phone from 54% to full, read 3.77volts after removal from device where as when fully charged they are 4.2 volts.

I am not going to use the micro USB input on this device to charge the 18650 cell though, as I do not trust cheap chargers to safely charge lithium cells.  It would be Neat if it could be trusted. Perhaps i'll test it and see what the termination voltage is and monitor it for heat during recharge.

There are multiple cell version of these type of USB power supplies, I chose the single cell version not only for the smaller size, but lithium cells in series or parallel, or both, need to be very very close in voltage or bad things can happen.  Meaning do not treat them like standard Alkaline or Nimh or Nicad batteries and mix charged and discharged batteries.

Please take care with Lithium batteries and understand the dangers before extracting cells from old laptop battery packs.  They can really pack a punch if shorted out, and can be detrimental to one's health if the worst happens

I do not know how well a single cell version would work on a tablet or Ipad which  can require higher amperages to charge properly, or perhaps at all.

I'll update as I acquire more data and experience with this new toy.
18650's are great batteries.  I had a backup laptop battery from the company I use to work for.  It had never been used and I do not know when it was manufactured.  When I took it apart the 18650's had extremely low voltage and I could not get them to take a charge.  Wandering if some kind of circuit may have drained them or what might have happened.

I dropped my main carry light a Nitecore SRT5 and now the main LED no longer burns.  I have several Nitecore lights and now don't have faith in there durability.

I just replaced the SRT5 with a Fenix PD35 and it can use flat or button top 18650's.  Guess most of my lights are all Chinese made and priced low enough but definitely need to have backups.
I am not sure of the exact voltage, but I believe if an 18650 cell is taken below 2.2 volts, one should not attempt to recharge it. 3 of my cells read 3.7, the other 6 read 4.1 when I extracted them.  All but the one cell held 4.2v for a long time after charging them individually on the Nitecore I4v2.

 I have a Nitecore HC50 light that I very much like.  It has taken some good abuse  and given me good use without issue.  My buddy just bought a '900 lumen' Fenix light, and I was not impressed with either the light output or plastic feel/weight of the unit.  I had always heard good things about Fenix and was surprised at my initial impression of it.  Not sure exactly which model he bought, but for 90$ I was unimpressed.

I've got some higher capacity Nitecore 14500 protected cells I use in a Smaller LED light that has a very tight beam when focused so, and they perform well.  The protection circuit kicks in surprisingly early during discharge though. It shows I was overdepleting the horrid unprotected cells that came with the light, or the charger, I can't remember how I got my first 14500 cells, all I know is I dropped them in the Bestbuy Liion recycling bin soon as I got the protected Nitecores.

About the product that is the subject of this thread, I did use it to recharge the Panasonic cell which just charged my phone, and monitored the voltage closely as it charged, and it did Stop right at 4.2 volts, Which made me happy.

 The USb power meter showed it was consuming a maximum 0.88 amps at 5.12volts during recharge. I think this rate is higher than my Nitecore I4v2 can do with just a single 18650 plugged into it. 

Now I do not know if every unit delivered will stop charging exactly at 4.2 volts as it should, all I can say is mine can, and I will likely buy a few more and perhaps gift them with my laptop extracted cells within to friends/ acquaintances I know who often find themselves with dead android  phone batteries.  Twelve 18650 cells is more cells than I can realistically make use of, and 4$ is hardly a bank breaker.  the 30 day delivery is more disappointing than anything, so far.
Oh, BTW, my Nitecore HC50 sometimes does not work with flattop cells. I used my new 140 watt soldering gun to apply a Blob of solder to all my flattop cells so that this is no longer an issue:

Came out pretty good IMO
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I ordered 2 more of these units, in the red and blue colors, with the original being black and they arrived in about 10 days.

I found that the new ones were able to provide the maximum 0.97 amps expected to my phone, but only with my healthiest unprotected 18650 battery and only when it was fully charged, and not for to long. As soon as the loaded output voltage dropped to 14.70, then the maximum amperage dropped to 0.88a, and then voltage rose back upto the 4.88v range.

These units when powering USB devices emit a Solid Blue LED light.

These devices, when charging the 18650 Cell inside via a USB port, have a blinking red LED light, which goes Solid when the battery stops charging.

All 3 of these stop before 4.2V, just under, which is fine and actually better for the 18650 Cell to be slightly undercharged.

However the newly Blue unit, the red LED does not function. I only knew it was charging because of my USB power meter was indicating it was passing current. The battery came out at 4.18V.

So of the 3 units I ordered, one has an issue, but is still task functional in that it both powers USB devices and can also charge the battery within from a USB source, there is just no visual indicator that it is charging the 18650 cell.

One other thing to note is I am now using Shorter 1 foot long micro USB cables. Since these have less wire, there is less voltage drop, and it seems to charge the phone faster. My USB meter reads the same maximum amperage, but if the voltage reaching the phone is higher, then so is the overall wattage into the battery.

I find the short cable to be more ergonomic in my usage too.
My Nitecore HC50 came with 2 CR123 disposable batteries.  I put those in my Old Film SLR instead, and use a single 18650 in the HC50.