I have this same 'Battery Low' issue where it fires for about a second and then gives battery low even on fully charged battery. I have a Smok Osub TC unit and it worked great when new but as time wore on and repeated change of battery the problem surfaced. In my case I believe I've tracked the problem to the battery chamber itself, specifically the tension supplied by the battery door is no longer sufficient to maintain a good connection. I found that if I apply manual pressure upward on the door and hold it the unit fires normally. But who wants to be squeezing the battery chamber to get contact just to vape! I haven't found a solution bu am considering an insert disc like a penny or something where I can get the door closed and still get good contact. If anyone has come across this and have a better solution, I'd like to know what you did.