The AAA and 10440 batteries are the same size, but not the same voltage. AAA batteries in alkaline, lithium or Ni-MH are rated at 1.5 volts, where the 10440 is Li-ion and is 3.7 volts. Using a AAA battery in place of a 10440 will often work, with lower overal output. Using a 10440 in place of a AAA will often just destroy the device as the voltage will be too great. Only use 10440 batteries in devices that are designed to handle 3.7 volts.