I have a cheap $20 fandyfire 502B single mode china light that uses a single 18650 battery and a XM - LT6 cree that is labeled 3.7-4.2 volts on the reflector. For some reason (yes, I know it is cheap china), the first time I turned it on the light went on for 5 seconds and then off and I can not get it to work. I have tried taking the battery and simply putting it to the +/- part of the springs and it does not work (circumvented the tube and tail switch). The 18650 is charged and reads 4 volts with my volt meter. If I quickly touch a 9 volt to the +/- the light will go on. What is the issue here and what can I do short of sending it back (the seller will take it and pay for it but I figured if I can fix it it will give me a challenge and I won't have to hassle with the post office. Thanks folks.