Picked up a Goal Zero flashlight from Woot last week, and was upset to find that (contrary to the advertisement) it won't charge normal 18650 cells.
The battery it came with was unusual, in that it had two contacts on the top of the cell which corresponded with two contacts inside the flashlight:
These charging pins, when powered using the USB port, put out a positive and a negative value which combined make ~4.2v.
(Note: I switched the alligator clips accidentally - when keeping the same reference one of the readings should be negative)
Seems like that secondary contact ring on the battery is just routed to the negative contact of the battery, which prevents the flashlight from charging any battery other than the Goal Zero branded ones. I called the company, and they confirmed my supposition (Yay American companies with good customer service and accessible engineers!).
Since I have a bunch of old laptop batteries from work (which, incidentally, is why I care about being able to charge standard 18650s), I decided to use some more bits of it for this project - I remembered that these batteries very seldom use actual wires, and more often use flat metal strips coated in Kapton for connections, which is great since I don't have a lot of room to play with inside the flashlight body. Grabbing a knock-off exacto knife, I managed to grab this without bleeding out on the workshop floor (this time).
I removed the covering from the outer charging pin and attached the connector to the spring inside using the poor man's solder. (I wanted to stick with non-permanent modifications until I was sure I wouldn't set my house on fire)
I reassembled the charging pill
And put it back in its housing
That's pretty much it, really. The flashlight goes back together as you'd expect, except there's a thin connector that sticks out the bottom and gets sandwiched between the 18650 cell and the spring on the tail cap. The charging circuitry has battery protection built in, so this flashlight can handle unprotected 18650s. I've charged several unprotected cells, and can confirm that the light starts charging and stops around 4.17v or so. According to the engineer, the charging circuitry is limited to ~500mA, which is unfortunate, but for ~$20 I don't mind.
Enjoy your light, and don't burn down your house!