I had a Ryobi 18V lithium pack that wouldn't charge. It got wet outside in a flash rain storm while building our barn. I took it apart and found the large I.C. chip had evidence of the magic smoke. Upon inspection I found all the cells had over 3.75 Volts. 10 of them to be exact.
So having 10 18650 batteries and no charger I started to investigate and found this video. I built it and it works perfectly. I use a USB to Micro usb to power the first board and the other 4 boards are fed from that.
Not my video...
I bought 10 of those little charge boards for $1.99 and I think $1.99 shipping. From ebay seller
onfine2008 . I used 4 and have 6 left. So I took a solar panel that puts out 6 volts in bright sunlight from a harbor freight 36 led security light. Our horse beat the light with a stick and we never found it, It had a lithium battery!! I saved the solar panel that was mounted outside the barn. I haven't used it to try and charge a battery yet as it's overcast. But in the little sun we have today it lit the blue l.e.d. on the board saying it is ready to charge. I'm sure it will only charge ONE 18650 but if the sun can do it, it only cost me two pieces of trash and a $0.20 circuit board.
My goal is to get a better flashlight for the 18650. Right now I'm using a $20 atomic beam from walmart.