Your batteries have to put out enough amps without sagging below the driver cutoff, you could go to 3 cells of good quality for short duty cycles, if you are using 2 x 16340 then your voltage is probably sagging below your drivers cutoff so you can't get the full 7.8 watts out of the diode. 2 LG INR 18650's will work, or you could try 3 AW 16340's