I am using the Premium 100 pack.
I've noticed that on occasion, the battery doesn't charge, regardless of the pack charge level. This happens most frequently immediately upon placing the dead battery into the charging port. For some reason, the battery charge feature is triggered by the opening and closing of the pack. I'm assuming this, because a full charged battery will sometimes begin charging again (red light blinking on the pack) for a few minutes after opening the pack to pull out a new flavor cartridge.
If the red lights don't begin to flash (indicating charging of the battery) when you put a drained battery into the pack, simply wait a few minutes and then reopen the charging pack lid, and then close again. I do this until the red charging lights begin to blink.
I'm speculating the non-charge issue has something to do with a fully discharged battery still being warm (put into charging pack immediately upon it going dead).
Just reopen & close the pack until charging begins. Works for me . . .