The operating voltage of white LEDs coupled with my emotional need for low cost for a given function, motivates me to make LED drivers suitable for battery operated lanterns that can run from a small number of cells. White and Ultraviolet LEDs available at the time of this writing, require higher voltages than colored LED`s. In the case of the ones I bought most recently, from 2.8 to 4 volts, depending upon operating current and the individual device.
A 006P 9 volt transistor radio battery can drive a 4 volt LED quite easily if the correct resistor is placed in series. The 9 volt transistor radio battery is among the most expensive of the common primary batteries, and has a rather low energy capacity,