Almost all inexpensive commercial LED flashlights use a 4.5V power supply—three AA or AAA batteries—because white LEDs require 3.3 to 3.5V to fully turn on. Thus ...
3V is sufficient to turn on any LED, but is within its reverse break down voltage rating (that is why I did not use the common 9V battery). Short circuit current is approx. 30mA. I used a 3V lithium ...