The AA and 14500 batteries are the same size, but not the same voltage. AA batteries in alkaline, lithium or Ni-MH are rated at 1.5 volts, where the 14500 is Li-ion and is 3.7 volts. Using a AA battery in place of a 14500 will often work, with lower overal output. Using a 14500 in place of a AA will often just destroy the device as the voltage will be too great. Only use 14500 batteries in devices that are designed to handle 3.7 volts.