A simple question. Usually I (we) unscrew the cap of the flashlight a little bit to prevent parasitic discharge of the battery when the flashlight isn’t used. Sometimes I forget and charge the flashlight via the USB port, and it seems to me that most flashlights can even be charged in this state. Should we always screw the cap on completely before charging?
Which flashlight models are you referring to?
The USB Rechargeable lights that I have, cannot charge unless the tailcap is screwed on tight as they do not have a connection to the negative side of the battery.
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Yeah, that’s what I was thinking. 
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At least Wurkkos FC11 and FC12. It is possible that the charging process does not really take place, but the red-green charging status is displayed correctly.