Why add a resistor? The internal resistance of a 3V coin cell is enough to keep the LED from blowing. I always keep a coin cell around to test LEDs before they go in to projects. The question is, how much current are they drawing. Here is probably the biggest revelation for me on this project:
The lower the LED voltage, the higher the current it will draw! I know you EEs are ready to hand out a dope-slap, but hey, there it is. When I tested a plain old yellow 2V LED with it was pulling more than 40 mA, way more than healthy! A higher voltage LED like blue or white would only pull 10 mA. Still, at 10mA, how long is a 225 mAH battery going to last? Yes, about a day. We still need a resistor.
- Wrap one end of the 330-ohm resistor around the LED's short lead, solder it and clip it.