Re: ATM batteries for LED

["Dave McCarter" <dmccarter@sympatico.ca>]

> The desire for max life of the battery is more the reason for the 9V
battery

That is a false economy. Lets look at the math.

Suppose the battery is 9.3 volts new. The LED requires 2.3 volts and draws
10mA.
There is a 9.3-2.3 = 7 volt drop across the resistance.
The LED produces 2.3v x 0.01A = 23mW of light and heat.

The resistance produces 7v x 0.01A = 70mW of heat, which unless the mirror
testing room is cold, is total waste. The circuit is only 23/93 or about 25
percent efficient. It is true that as the battery voltage drops if the
resistance can be lowered to keep LED current constant the resistance will
dissipate less and less power and total efficiency will improve. But most
battery discharge curves are pretty steep near the end of service, so this
is overall not very important. By the way, a nine volt battery is made up of
cells with quite low current capacity, so life length is short.

What if the battery choice is a pair of AA cells with 4 to 5 times the
current capacity? The new potential is 1.65 x 2 = 3.3v, so the drop across
the resistance will be 1 volt, and the heat produced will be 1v x 0.01A =
10mW. Again, if the resistance is variable the LED will continue to operate
until the batteries get down to just over 2.3 volts, or 1.15 volts each. Now
that is a dead AA battery, and through it's life 23/33 or 70% of the energy
was delivered to the LED. And the AA cells will last many times longer.

Dave