On Thu, 16 Oct 2003 17:21:36 GMT, (Gary Coffman)
wrote:

They must. LEDs don't have the thermal lag that filament type bulbs have.
So they can pulse the LED with a reasonably low duty cycle. If they use
a 5% duty cycle, they can get average current draw down to 2.2 mA while
still producing the same peak output as running continuously at 44 mA.

Of course *average* output is also only 5%, but the eye has persistence,
and is fooled into thinking it is seeing a much brighter light. Some strange
effects would occur for rapidly moving objects, strobing, but for nearly
stationary scenes you'd see about as well as if they were actually drawing
40 mA.

This is also more efficient than using a current-limiting resistor.
They may limit current with a small inductor, and vary duty cycle as
battery voltage decreases so as to run at constant intensity until the
battery is depleted even somewhat below the usual 0.8 volt per cell
that is regarded to be "dead".

The Everready X95 D-cell is rated at 18900 mah -- yes, 18.9 amp hours.
For an average voltage of 1.2 volts, this is 22.6 watt hours per cell,
or 68 watt-hours for 3 cells. (Gunner, the C-cell has about half that
capacity.) 6 LED's running at 40 mA and 2 volts each would consume
0.48 watts, so (not counting circuit inefficiency) the batteries could
last 141 hours with the right circuitry and deliver full brightness
right to the end. Efficiencies of well over 90% are quite
possible with simple electronics. The drop in a 20 milliohm MOSFET at
240 mA would be something like .005 volts! Synchronous control chips
(using MOSFET synch rectification) are now available that will operate
on 3.3 volts and maybe as low as 1.5 volts.