What I normally do I leave them in the head torch.

Approximately every two weeks when I have night shift work I charge them while driving to work

Last week while I was driving to the work night shift I used USB cable supplied with head light and I drove for about 2 hours (means batteries on charge for 2 hours)

while I was at work I tried to use the head light nothing tuned on !!!! hooked head torch with USB cable there is light once I unplugged no light!!! same happened before two months with different set of battries

When I came home and used my USB Nitecore UM20 charger and charger read zero means batteries not getting charged

Regards