I'm planning on adding leds to a small quadcopter which runs on a 1s lipo battery. I am planning on attaching the leds straight to the battery rather than the regulated voltage the main circuit board uses. The battery when fully charged has a voltage of 4.2 volts and a nominal voltage of 3.3.
Assuming an led with a 2.5v forward voltage and a 20Ma forward current. the calculations at either range result in either a 100ohm resistor (for 4.2 volts) or 47ohm resistor (for 3.3 volts). What size resistor should I use? should I use the 100 ohm resistor and let the brightness dim towards the end of the battery life?
In general what is the best way of selecting components where the system is not complex enough to warrant using a voltage regulator?